Possible deadlock in live query callback

https://github.com/couchbase/couchbase-lite-C/issues/209

C4Query::liveQuerier breaks the rule about calling external code while holding an internal lock. As a result there is a situation where via convoluted circular mechanisms the code can end up back in C4Query and attempt to seize the lock again, resulting in a deadlock:

• C4Query::liveQuerierUpdated seizes lock
• C4QueryObserverImpl::notify
• ListenerToken observe callback
• ListenerToken::queryChanged
• CBLDatabase::notify
• Above function retains the listener token passed into it
• All other areas in the meantime release the listener token (ref count = 1)
• At the end of notify listener token is released to 0
• ~ListenerToken<CBLQueryChangeListener>()
• ListenerToken<CBLQueryChangeListener>::setEnabled
• C4QueryObserverImpl::setEnabled
• C4Query::enableObserver deadlock

Recommendation:  Call the observe methods without holding the lock, if possible