Add Estimated Number of Keys Function

[alexanderkiel]

We need this functionality for all key-value instead of only for RocksDB because we will use it in the CQL expression cache.

Needed in: #1051

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.19%. Comparing base (4497165) to head (994694d).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1817   +/-   ##
========================================
  Coverage    96.19%   96.19%           
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  Files          271      271           
  Lines        17342    17350    +8     
  Branches       347      347           
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+ Hits         16682    16690    +8     
  Misses         313      313           
  Partials       347      347           

Files	Coverage Δ
modules/admin-api/src/blaze/admin_api.clj	98.03% <100.00%> (ø)
modules/kv/src/blaze/db/kv.clj	100.00% <100.00%> (ø)
modules/kv/src/blaze/db/kv/mem.clj	100.00% <100.00%> (ø)
modules/kv/src/blaze/db/kv/protocols.clj	100.00% <ø> (ø)
modules/rocksdb/src/blaze/db/kv/rocksdb.clj	99.36% <100.00%> (+<0.01%)	⬆️