add a thread-dump on signal handler

[ara4n]

i've just had to mess around with a dbg python & py-bt to try to get a thread dump out of synapse. i wish i could have just hit it with a signal (which would work even if the manhole was unavailable for whatever reason) - something like https://stackoverflow.com/questions/132058/showing-the-stack-trace-from-a-running-python-application/2569696#2569696

[ara4n]

nb it would need to log to stderr to bypass logging failures, and so synctl would need to send stderr somewhere useful (at last)

[richvdh]

so synctl would need to send stderr somewhere useful

cf #1539