The tests for GasFeeController are currently lacking in a few ways:

* `nock` is being used to test that requests are being used to obtain
  data; however, GasFeeController takes functions as arguments which can
  be used to stub out those requests, thereby removing the need for
  `nock`.
* The logic within `getGasFeeEstimatesAndStartPolling` is not being
  fully exercised, specifically the polling code.
* There are no tests for `disconnectPoller`.
* There are no explicit tests for `stopPolling`.
* The logic within `_fetchGasFeeEstimateData` is not being fully
  exercised, specifically with regard to  how the result of `getChainId`
  changes the URL that ends up being hit for both EIP-1559 and
  non-EIP-1559 flows.
* There are tests categorized under a `getChainId` describe block which
  are actually for `_fetchGasFeeEstimateData`.

This commit attempts to address these issues so that future changes to
GasFeeController do not break existing behavior.

We would like to introduce another fallback to how we estimate gas fees
in a future commit — namely, we want to use `eth_feeHistory` for
EIP-1559 networks if the usual API is down. To do this, the existing
code around gas fee estimates needs to be refactored. This is the first
in a two-part refactor, extracting the majority of the code in
`GasFeeController._fetchGasFeeEstimateData` to `gas-util`. In addition
to making room for more changes, this also allows us to test this code
in isolation from the polling code that already exists in
GasFeeController.

Note that I am using the term "suggestions" to encompass the data that
`_fetchGasFeeEstimateData` returns. This is because `gasFeeEstimates` is
an intermediate value that can also be packaged with
`estimatedGasFeeTimeBounds`, so "estimates" could be confusing. Also, I
replaced "fetch" with "determine" as in the future we may be performing
calculations that APIs have done for us in the past ("fetch" seems like
a more low-level action).

The tests for GasFeeController are currently lacking in a few ways:

* `nock` is being used to test that requests are being used to obtain
  data; however, GasFeeController takes functions as arguments which can
  be used to stub out those requests, thereby removing the need for
  `nock`.
* The logic within `getGasFeeEstimatesAndStartPolling` is not being
  fully exercised, specifically the polling code.
* There are no tests for `disconnectPoller`.
* There are no explicit tests for `stopPolling`.
* The logic within `_fetchGasFeeEstimateData` is not being fully
  exercised, specifically with regard to  how the result of `getChainId`
  changes the URL that ends up being hit for both EIP-1559 and
  non-EIP-1559 flows.
* There are tests categorized under a `getChainId` describe block which
  are actually for `_fetchGasFeeEstimateData`.

This commit attempts to address these issues so that future changes to
GasFeeController do not break existing behavior.

…-recommendations

The tests for GasFeeController are currently lacking in a few ways:

* `nock` is being used to test that requests are being used to obtain
  data; however, GasFeeController takes functions as arguments which can
  be used to stub out those requests, thereby removing the need for
  `nock`.
* The logic within `getGasFeeEstimatesAndStartPolling` is not being
  fully exercised, specifically the polling code.
* There are no tests for `disconnectPoller`.
* There are no explicit tests for `stopPolling`.
* The logic within `_fetchGasFeeEstimateData` is not being fully
  exercised, specifically with regard to  how the result of `getChainId`
  changes the URL that ends up being hit for both EIP-1559 and
  non-EIP-1559 flows.
* There are tests categorized under a `getChainId` describe block which
  are actually for `_fetchGasFeeEstimateData`.

This commit attempts to address these issues so that future changes to
GasFeeController do not break existing behavior.

…-recommendations

If we are on an EIP-1559-supported network and the Metaswap API fails
for some reason, fall back to using `eth_feeHistory` to calculate gas
estimates (which the API uses anyway). This code is more or less taken
from the code for the API ([1]).

[1]: https://gitlab.com/ConsenSys/codefi/products/metaswap/gas-api/-/blob/eae6927b1a0c445e02cb3cba9e9e6b0f35857a12/src/eip1559/feeHistory.ts

When a user is about to send a transaction or is looking at a swap
quote, we would like to inform them if the network is busy so that we
can push them to use a lower fee setting. GasFeeController already
provides a way to poll for fee estimates, which we employ on transaction
preview screens. This commit updates the polling code so that we also
gauge network status as we pull estimates. This is done by hitting
another endpoint in the MetaSwap API which specifically gives us a base
fee threshold we can use to determine whether the network is busy
(falling back to using `eth_feeHistory` if the API is not available).