[java-bigtable] Add idle channel eviction for the channel pool

[nicholsl]

[lcian]

Hi, I believe that this has been implemented in googleapis/java-bigtable#2651 and this issue can be closed.
I would still like to ask, could you please provide more context on why this change was needed in the first place?
What was the issue observed in practice that this should solve or mitigate?

[lcian]

@igorbernstein2 maybe you have context around this?