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.