Unload obsolete memtries post block processing

[staffik]

Currently we remove obsolete memtries only when we start state sync for a new shard: https://github.com/near/nearcore/blob/master/chain/client/src/client.rs#L2507-L2519
However, it does not unload obsolete memtries in all cases.
For example, if we tracked shards [0, 1, 2], and then we track shards [0, 1] only, shard 2 won't be unloaded, because we did not start a new state sync.
Although it is not critical, trie for shard 2 could occupy memory forever.

This PR calls the routine to unload obsolete memtries as a block post-processing step too.

[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.61%. Comparing base (74ac5fe) to head (f58c2a1).
Report is 3 commits behind head on master.

Files	Patch %	Lines
chain/chain/src/chain.rs	91.30%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11657      +/-   ##
==========================================
- Coverage   71.65%   71.61%   -0.05%     
==========================================
  Files         787      787              
  Lines      161063   161104      +41     
  Branches   161063   161104      +41     
==========================================
- Hits       115413   115373      -40     
- Misses      40597    40684      +87     
+ Partials     5053     5047       -6     

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[Longarithm]

That sounds right. However, still looks hacky that we are doing this check for every single block, while the shard "status" changes only once per epoch.

I have a feeling that we can call retain_mem_tries only if epoch_manager.is_next_block_epoch_start(parent_hash) is true, and remove retain_mem_tries from run_catchup because it doesn't fit there. Would you like to try it?

Also nit - if the function is called retain_mem_tries, the comment should also explain why retaining happens. The fact that it mentions GC (the opposite event) is confusing.