Manually backported the subject CLs, because of lack of
Gerrit "forge-author" permissions, but also because the prior
cherry picks didn't apply cleanly, due to a tight relation chain.

The backport comprises of:
* CL 174122
* CL 216197
* CL 223963
* CL 216240
* CL 216241

Note:
Due to the restrictions that we cannot retroactively
introduce API changes to Go1.14.6 that weren't in Go1.14, the Conn.Validator
interface (from CL 174122, CL 223963) isn't exposed, and drivers will just be
inspected, for if they have an IsValid() bool method implemented.

For a description of the content of each CL:

* CL 174122:
database/sql: process all Session Resets synchronously

Adds a new interface, driver.ConnectionValidator, to allow
drivers to signal they should not be used again,
separatly from the session resetter interface.
This is done now that the session reset is done
after the connection is put into the connection pool.

Previous behavior attempted to run Session Resets
in a background worker. This implementation had two
problems: untested performance gains for additional
complexity, and failures when the pool size
exceeded the connection reset channel buffer size.

* CL 216197:
database/sql: check conn expiry when returning to pool, not when handing it out

With the original connection reuse strategy, it was possible that
when a new connection was requested, the pool would wait for an
an existing connection to return for re-use in a full connection
pool, and then it would check if the returned connection was expired.
If the returned connection expired while awaiting re-use, it would
return an error to the location requestiong the new connection.
The existing call sites requesting a new connection was often the last
attempt at returning a connection for a query. This would then
result in a failed query.

This change ensures that we perform the expiry check right
before a connection is inserted back in to the connection pool
for while requesting a new connection. If requesting a new connection
it will no longer fail due to the connection expiring.

* CL 216240:
database/sql: prevent Tx statement from committing after rollback

It was possible for a Tx that was aborted for rollback
asynchronously to execute a query after the rollback had completed
on the database, which often would auto commit the query outside
of the transaction.

By W-locking the tx.closemu prior to issuing the rollback
connection it ensures any Tx query either fails or finishes
on the Tx, and never after the Tx has rolled back.

* CL 216241:
database/sql: on Tx rollback, retain connection if driver can reset session

Previously the Tx would drop the connection after rolling back from
a context cancel. Now if the driver can reset the session,
keep the connection.

* CL 223963
database/sql: add test for Conn.Validator interface

This addresses comments made by Russ after
https://golang.org/cl/174122 was merged. It addes a test
for the connection validator and renames the interface to just
"Validator".

Updates #31480
Updates #32530
Updates #32942
Updates #34775
Fixes #39101

Change-Id: I043d2d724a367588689fd7d6f3cecb39abeb042c
Reviewed-on: https://go-review.googlesource.com/c/go/+/242102
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>