Go SDK-Tune in memory pre-combine caching for Lifted Combines.

Requires [BEAM-4276](https://issues.apache.org/jira/browse/BEAM-4276) to be completed first.

Additional performance tweaks to the in memory per-key accumulator cache during the pre-combine phase of a lifted combine.

This can include any of
* capping the number of key-accumulator in the cache, and draining them eagerly after x elements seen, or evicting keys by some heuristic after the cache has seen X distinct keys.
* providing a counter on cache size (key/element counts) exposable through the metrics interface or another runner standard counter, to permit observing the cache's status, especially if it could grow without bound within a bundle.

Imported from Jira [BEAM-4468](https://issues.apache.org/jira/browse/BEAM-4468). Original Jira may contain additional context.
Reported by: lostluck.