core: protect against popping a message off the queue if no handler is attached.

This mostly solves a problem with threads being launched in random
order, so a handler might not be established yet, even if there is a
message waiting. Fixes Issue #514.
Merge branch 'maint'

Conflicts:
	gnuradio-core/gnuradio-core.conf
core: moving over setting of max_messages option to conf.in file.

blocks: these timers were actually doing much, so we can speed up the tests.