upipe_xfer: use ueventfd to suspend remote thread - #1227

Using a mutex for blocking a remote event loop may introduce unbounded
wait because pthread_mutex_unlock does not guarantee that another thread
waiting to acquire the lock will be scheduled right away.

Instead, use a pair of ueventfd for synchronization, and add a
UPIPE_XFER_YIELD message to ask the remote thread to suspend itself.

Moreover, mutex based exclusion was generally impossible to implement in
upump managers other than libev.