Use connection pool in PostgresStore

[chasestubblefield]

This allows PostgresStore to be used in a threaded environment like with the puma web server.

An optional Hash with pool options can now passed when configuring the store:

config.content_delivery = :eager_sync, :postgres, ENV['POSTGRES_CONNECTION'], {size: 5, timeout: 5}

The default is the same as the connection_pool gem, {size: 5, timeout: 5}.

Also, the connection pool is available through PostgresStore#connection_pool to allow clearing active connections before forking, useful with puma or unicorn.

I think this is a cleaner approach than #69.

[chasestubblefield]

@gburgett Addressed your concerns about Query and added a test that uses threads, would appreciate your feedback. CI failures seem to be caused by latest Rubocop version.

[watermarkchurch-bot]

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[coveralls]

Pull Request Test Coverage Report for Build 439

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-2.0%) to 93.428%

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Files with Coverage Reduction	New Missed Lines	%
spec/support/fixtures_helper.rb	7	46.15%
<!--	Total:	7		-->

Totals
Change from base Build 429:	-2.0%
Covered Lines:	526
Relevant Lines:	563

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💛 - Coveralls