Add expiration time in preparation for the TTL feature.

TTLs are based on expiration in the LRU - during getting if the value is
expired then it is removed from the LRU and we return nothing. This is
needed to have short-lived values in a distributed system. To achieve
this, expand the `valWithStat` struct with `expire time.Time`. Then,
expand all of the related interfaces:
- Codecs
- `setLRUOnEvicted`
- etc.

Since the HTTP interface returns in the body raw bytes, send the
expiration timestamp with headers. Expand the test in
`http/http_test.go` to test whether the keys are being regenerated after
the timestamp. Copy over a test from mailgun/groupcache for TTLs in the
LRU.

*: add TTL functionality

Upgrade internal interfaces for multi-key fetch:

- Use multi-part HTTP request/response for retrieving data about
multiple keys. Boundary is randomly generated by Go standard library so
this seems like the best path;
- Upgrade gRPC interface to have `repeat string Keys` instead of `string
Key`. `key` has priority over `keys` but users should use `keys` either
way.

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>

*: prepare for multi-key fetch

Switch to Go 1.17 && enable golangci-lint.

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>

.github: update workflows

* Add `ValueWithTTL` struct for grouping value with time.Time expiration
time;
* Update interfaces to use it;
* Add stub for multiple key retrieval;
* Fix golangci-lint errors.

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>

*: update interfaces