database/sql: Fix idle connection reuse

Fix idle connection reuse so that ConnMaxIdleTime clears down excessive
idle connections.

This now ensures that db.freeConn is ordered by returnedAt and that
connections that have been idle for the shortest period are reused
first.

In addition connectionCleanerRunLocked updates the next check deadline
based on idle and maximum life time information so that we avoid waiting
up to double MaxIdleTime to close connections.

Corrected the calling timer of connectionCleaner.

Fixes #39471

Change-Id: I6d26b3542179ef35aa13e5265a89bc0f08ba7fa1
Reviewed-on: https://go-review.googlesource.com/c/go/+/237337


Reviewed-by: Tamás Gulácsi <tgulacsi78@gmail.com>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Trust: Ian Lance Taylor <iant@golang.org>