Transient errors inside transaction unpins the session.
Add brief docs about sharded transactions and add 3.9 changelog.
Tests changes:
Add spec tests for sharded transaction recoveryToken.
Speed up txn tests by reducing SDAM waiting time after a network error.
Remove outdated test workaround for killAllSessions.
Database.command without a read_preference defaults to the
transaction's or Primary if the session is not in a transaction
Add runCommand transaction tests.
Update count tests.
