If wait_for_conn is dropped before completing, release will call
wake on a waker that noone is listining on. This leads to a state where
waiting will gro indefinitly while all connections are idle.
To fix this we turn waiting into a queue of unique Weak pointers.
This way if wait_for_conn is dropped the pointer in waiting cannot
be upgraded, we can use this as a signal that we shoud wake the
next one instead.
c7cf104a8f