idl: Gracefully handle destroying a transaction before receiving its reply.

idl: Gracefully handle destroying a transaction before receiving its reply.

If ovsdb_idl_txn_destroy() is called to destroy a transaction before its
reply has been received from the database server, then until now we would
drop the connection to the database when the reply actually arrived,
because we would have no record of that transaction ID any longer.

Notably, ovs-vswitchd does this: it "fires and forgets" database
transactions.  (Really, it should not do that, but that's a bigger commit.)

This commit fixes the problem by not dropping the database connection in
such a case.

This fixes an observed problem such that sometimes ovs-vsctl took a long
time to complete, which was because ovs-vswitchd was dropping its
connection to the database and backing off.