feat(NODE-2993): implement maxConnecting

[dariakp]

Description

NODE-2993

What is changing?

• Connection pool can now attempt to check out up to maxConnecting number of connections in parallel (previously it would only establish the requested connections one at a time)
• ensureMinPoolSize will now only attempt to create one connection at a time so as not to block checkout requests from obtaining connections as quickly as possible (since ensureMinPoolSize always adds the connections to the available pool first, a checkout request would otherwise have to wait to retrieve it from the pool instead of obtaining it immediately; this nonblocking behavior is relied upon by the "threads blocked by maxConnecting check out minPoolSize connections" spec test)
• The ConnectionCreatedEvent is now fired before actually establishing the connection, as required by the spec (the maxConnecting tests rely on this to differentiate between pending and established connections)
• There was a bug in the cmap runner where the operations weren't actually queued using the main thread but rather in parallel across threads, this PR includes a fix for that
• Note: the new waitQueueTimeoutMS test is skipped because it is not applicable given our waitQueueTimeoutMS implementation (see the corresponding test yaml file for the relevant note)

Is there new documentation needed for these changes?

No

What is the motivation for this change?

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: <type>(NODE-xxxx)<!>: <description>
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[durran]

Seems to be working as expected for me locally. Think this is good to go.