Reduce wait for validators who are offline, remove mem leak

[ilblackdragon]

Fixes #1639

Additionally, replacing HashMap in EpochManager with SizedCache to limit memory requirements.

[codecov]

Codecov Report

Merging #1647 into staging will increase coverage by 0.15%.
The diff coverage is 97.27%.

@@             Coverage Diff             @@
##           staging    #1647      +/-   ##
===========================================
+ Coverage    79.74%   79.89%   +0.15%     
===========================================
  Files          130      130              
  Lines        23923    24042     +119     
===========================================
+ Hits         19078    19209     +131     
+ Misses        4845     4833      -12

Impacted Files	Coverage Δ
chain/chain/src/types.rs	97.29% <ø> (ø)	⬆️
core/primitives/src/transaction.rs	79.45% <ø> (ø)	⬆️
near/tests/tx_propagation.rs	95.5% <0%> (-1.05%)	⬇️
chain/client/src/client_actor.rs	82.77% <100%> (+0.69%)	⬆️
chain/client/src/types.rs	81% <100%> (+5.48%)	⬆️
near/src/runtime.rs	71.54% <100%> (+0.23%)	⬆️
chain/chain/src/test_utils.rs	78.99% <100%> (+0.13%)	⬆️
near/src/config.rs	94.82% <100%> (+0.06%)	⬆️
chain/epoch_manager/src/types.rs	76% <100%> (+20.15%)	⬆️
chain/epoch_manager/src/lib.rs	96.77% <98.88%> (+0.21%)	⬆️
... and 16 more

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