mysql/conn: do not allocate during writes

[vmg]

Description

This is an outlier in the profiles for OLTP benchmarks. There's a lot of wasteful allocations and CPU usage in the write path for the MySQL connection for two reasons:

1. We keep creating new Timers for flushing every write call. To fix this, keep a single Timer and call Reset.
2. We return a closure from getConn for every write call. The closure contains the cleanup required after the write (unlocking the buffer and triggering the flush), but it captures the connection pointer itself, so it keeps allocating. Fix this by inlining getConn and deferring the cleanup only when required.

Flame graph comparison (the size relative size difference is accurate):

Before (CPU)

After (CPU)

Before (Allocs)

After (Allocs)

cc @harshit-gangal @systay

Related Issue(s)

Checklist

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[vitess-bot]

Hello! 👋

This Pull Request is now handled by arewefastyet. The current HEAD and future commits will be benchmarked.

You can find the performance comparison on the arewefastyet website.

[dbussink]

Nice little improvement

[arthurschreiber]

❤️ This is a really nice improvement.