Fix disconnect all slots deadlock

[TolonUK]

Fixes a deadlock between concurrent calls to connection::disconnect() and signal::disconnect_all_slots().

In invalidate_disconnector(), the mechanism to wait for the destruction of _shared_disconnector gets stuck if a connection object has promoted its disconnector weak_ptr, has called disconnect(std::size_t) on the signal and is waiting to acquire the mutex.

This resolves this issue by separating the reset of _shared_disconnector from the wait for its destruction, so that the latter can be done outside of the mutex lock.

The problem behaviour was replicated in the test "Concurrent disconnection does not deadlock", prior to resolving the issue.