Call .flush once per command

[davidor]

This PR overrides Async::IO::Protocol::Line#write_line in Async::Redis::Protocol::RESP.

The original method performs a flush. This one does not and moves the responsibility of flushing to the caller of the method.

In the case of Redis, we do not want to perform a flush in every line, because each Redis command contains several lines. Flushing once per command is more efficient because it avoids unnecessary writes to the socket.

In a project that I'm working on, I noticed that, when using this library, a large % of the CPU time was spent on the write_lines method. Then I discovered that each call to that method performs a "flush" (https://github.com/socketry/async-io/blob/6b26891275d0b1b17f3edf5ce286c79c15066114/lib/async/io/protocol/line.rb#L45) which results in an IO.write (https://github.com/socketry/async-io/blob/6b26891275d0b1b17f3edf5ce286c79c15066114/lib/async/io/stream.rb#L141). After applying this patch, the % of CPU time spent on write_lines went down significantly.

[ioquatix]

Looks good.

[ioquatix]

I released in v0.3.3

[coveralls]

Pull Request Test Coverage Report for Build 69

• 7 of 8 (87.5%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 86.269%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/async/redis/protocol/resp.rb	7	8	87.5%
<!--	Total:	7	8	87.5%	-->

Totals
Change from base Build 68:	-0.01%
Covered Lines:	333
Relevant Lines:	386

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💛 - Coveralls