setting Pd's clocks is not thread-safe, but calling sys_lock() will
slow the entire process down to unusability. therefore, we use a
(per-receiver) thread, that get's waits on a pthread_cond_t and will
syslock if needed. LATER there should only be one of these clock-threads
(per RTE, in case we finally can run multiple Pd's in a single
applications), that maintains it's own list of callbacks+data.
tests for tcpclient/server: client disconnects -> server should get notified server disconnects -> client should get notified client crashes -> server should disconnect server crashes -> client should disconnect
[tcpclient] now does all the actual connect/disconnect work in a
helper-thread. unfortunately, iemnet__receiver_destroy() assumes that
it is called from the main-thread and will outlet_list the remaining
bytes in the dtor. however, outlet_list is not threadsafe. possible
solution: see speed&syslocks for a leightweight thread-safe callback
framework.