Fix possible writes to closed DCBs

When persistent connections were used, it was possible that the injection
of COM_CHANGE_USER statements caused a crash when a DCB in the wrong state
was accessed.

For MySQL protocol modules, the `data` member of the client DCB points to
the shared session data, a MYSQL_session struct, but for sessions in the
persistent pool, it points to NULL. The boolean, `was_persistent`, tells
whether a DCB was just taken from the pool or it has been in use.

The `was_persistent` status wasn't properly reset for connections that
were put into the pool which caused a COM_CHANGE_USER statement to be
injected for stale connections in the pool which caused a crash when the
NULL `data` member was accessed.

server/core/dcb.c

@@ -1827,6 +1827,7 @@ dcb_maybe_add_persistent(DCB *dcb)
         MXS_DEBUG("%lu [dcb_maybe_add_persistent] Adding DCB to persistent pool, user %s.\n",
                   pthread_self(),
                   dcb->user);
+        dcb->was_persistent = false;
         dcb->dcb_is_zombie = false;
         dcb->persistentstart = time(NULL);
         if (dcb->session)