Move the "pool" feature into external crate

[coveralls]

Pull Request Test Coverage Report for Build 590485362

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 49 unchanged lines in 7 files lost coverage.
• Overall coverage decreased (-0.9%) to 79.243%

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Files with Coverage Reduction	New Missed Lines	%
examples/reconnection.rs	1	4.17%
examples/insert.rs	2	3.33%
examples/namedcolumns_select.rs	2	6.67%
examples/select.rs	2	5.88%
examples/sys_infos.rs	2	8.33%
examples/string_conn.rs	3	5.0%
src/connection/builders/builder_native.rs	37	35.05%
<!--	Total:	49		-->

Totals
Change from base Build 588769270:	-0.9%
Covered Lines:	2848
Relevant Lines:	3594

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💛 - Coveralls