refactor query caching into a separate object to reduce code in Query…

[itsjoeconway]

… object.

This is the first part of a refactor to unify the execution context/connection interface to avoid duplicate code paths.

[codecov-io]

Codecov Report

Merging #39 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   91.19%   91.18%   -0.01%     
==========================================
  Files          11       12       +1     
  Lines         977      976       -1     
==========================================
- Hits          891      890       -1     
  Misses         86       86

Impacted Files	Coverage Δ
lib/src/transaction_proxy.dart	90.47% <100%> (ø)	⬆️
lib/src/query.dart	96% <100%> (ø)	⬆️
lib/src/query_cache.dart	100% <100%> (ø)
lib/src/connection_fsm.dart	86.86% <100%> (ø)	⬆️
lib/src/connection.dart	91.12% <100%> (-0.61%)	⬇️

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