MaxScale may hang with multiple concurrent maxadmin calls.

Some maxadmin calls, e.g. show dcbs, requires communication between the worker thread handling the maxadmin command and all other worker threads.

This is handled so that DCB information about the current worker thread (the one handling the maxadmin command) are collected directly and the DCB information about all other threads by sending a message via the epoll-mechanism to the other worker threads.

However, if multiple maxadmin commands are handled concurrently by different worker threads, this will lead to a deadlock as all but one worker handling a maxadmin command will sit in the maxadmin spinlock, thus not handling any messages sent to that worker.