Avoids a thread-safety race condition on shutdown where we could try to finalize a statement twice.
Allows the async thread to be referenced until ShutdownAsyncThread, so async finalizers can make use of it.
Removes the no more useful mAsyncExecutionThreadIsAlive.
Nullifies the sqlite3_mutex pointer when the connection is closed, since it would be a dangling pointer.
Use a ScopeExit to ensure the connection and the mutex pointers are always nullified on failure.
Makes asyncClose bailout early if a Close method was already invoked before.
Makes AsyncInitDatabase not use AsyncClose to just shutdown the async thread.
Fixes various unfinalized statements in consumers code.
Makes mConnectionClosed better follow mDBConn status.
Replaces some mutex locking isClosed() calls with lockless isConnectionReadyOnThisThread.
MozReview-Commit-ID: 6sftFehsQTt
