refactor(tcp_server): release state lock before sending packets

[kurnevsky]

Currently we send tcp packets while holding the state lock. At the
same time we use bounded queues for sending of packets. This means
that a single stuck client can block the whole server until the
timeout happens. This change makes the server release the state lock
before sending any packet.

[coveralls]

Pull Request Test Coverage Report for Build 2987516136

• 106 of 114 (92.98%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 95.99%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tox_core/src/relay/server/server.rs	103	111	92.79%

Totals
Change from base Build 2867531116:	0.008%
Covered Lines:	15489
Relevant Lines:	16136

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💛 - Coveralls

[kpp]

So basically this is release state lock + some refactoring. Looks good.