Implicit and Explicit transaction scope

[alex-dukhno]

Link with #574

Description

This PR adds:

• implicit transaction begin/commit
• explicit transaction begin/commit
• deallocation of PrepareStatement

[coveralls]

Pull Request Test Coverage Report for Build 814431988

• 120 of 180 (66.67%) changed or added relevant lines in 6 files are covered.
• 32 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.6%) to 64.756%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
node_engine/src/lib.rs	4	6	66.67%
node_engine/src/txn_context/mod.rs	2	4	50.0%
node_engine/src/worker.rs	60	73	82.19%
node_engine/src/query_engine_old/mod.rs	39	82	47.56%

Files with Coverage Reduction	New Missed Lines	%
node_engine/src/query_engine_old/mod.rs	3	49.39%
postgre_sql/query_parser/src/lib.rs	29	86.08%

Totals
Change from base Build 810855503:	-0.6%
Covered Lines:	2949
Relevant Lines:	4554

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💛 - Coveralls