coroutines: Use one global bottom half for CoQueue

Now that AsyncContexts don't exist any more, we can use one global bottom half
for restarting coroutines instead of allocating a new one every time (before
removing AsyncContexts, the problem with having a global BH was that it had to
belong to a single AsyncContexts and wouldn't be executed in a different one -
which leads to deadlocks)

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
