feature/faces: use pooler face instead pool struct; up coverage and init tests for BucketStat; add Stringer methods for rs info

[KaymeKaydex]

• use face instead pool struct
• add mocks and first test with mock for replicaset bucket stat
• make bucket stat public method(there is no problem if user wanna use it)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 16.20%. Comparing base (9e72b8c) to head (b2cf45c).

Files	Patch %	Lines
discovery.go	0.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #21 +/- ## ========================================== + Coverage 13.82% 16.20% +2.38% ========================================== Files 9 9 Lines 615 617 +2 ========================================== + Hits 85 100 +15 + Misses 525 511 -14 - Partials 5 6 +1 ```

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