better FFI::Cached->clear exceptions

	modified:   lib/POEx/ZMQ/FFI/Cached.pm
simple FFI::Cached test

	new file:   t/ffi/cached.t
delay write retry for 0.1s on EAGAIN/EINTR/EFSM

	modified:   lib/POEx/ZMQ/Socket.pm
fix return-if-retrying behavior in _pxz_nb_write

	modified:   lib/POEx/ZMQ/Socket.pm
clean up _pxz_nb_write

	modified:   lib/POEx/ZMQ/Socket.pm
clean up _pxz_nb_write

	modified:   lib/POEx/ZMQ/Socket.pm
minor _message_not_sendable optimizations

	modified:   lib/POEx/ZMQ/Socket.pm
$kernel->delay probably more sensible than $kernel->alarm

	modified:   lib/POEx/ZMQ/Socket.pm
minor test & example tweaks

	modified:   examples/ssh_tunnel.sh
	modified:   t/ffi/socket.t