Add YDB support

[floatdrop]

Initial draft of YDB support.

[coveralls]

Pull Request Test Coverage Report for Build 1618488104

• 130 of 219 (59.36%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 57.851%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
database/ydb/ydb.go	130	219	59.36%
<!--	Total:	130	219

Totals
Change from base Build 1595125260:	0.05%
Covered Lines:	3861
Relevant Lines:	6674

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💛 - Coveralls

[rekby]

@dhui, @Fontinalis

Is any comments/approve?

[floatdrop]

@rekby I think we should wait, until YDB will be ready to provide locking primitives.