Add db rollback on gevent timeout error

[varmar05]

Handle gevent exception when it occurs during db.session. Normally exception is propagated without any issues, however in case we need to do some cleanups (in finally clause) which involve db.session we need a special handling:

• a rollback to unblock cleanup
• reraise exception to be caught by middleware

[coveralls]

Pull Request Test Coverage Report for Build 13630944978

Details

• 24 of 36 (66.67%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 92.28%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
server/mergin/sync/public_api_controller.py	1	7	14.29%
server/mergin/tests/test_middleware.py	23	29	79.31%

Totals
Change from base Build 13584997311:	-0.1%
Covered Lines:	6801
Relevant Lines:	7370

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💛 - Coveralls

[MarcelGeo]

Just one thing in tests.

We do not have any other places in code with handling of psycopg errors? If not , ok :)