Improve signal-handling behavior of maestro-core

Document how threads in maestro behave wrt. signals sent to them.

Modify simple_pool_manager to ensure that signals for termination only reach the application logic and don't get delivered to the libfabric threads. libfabric/gni does not react well to SIGUSR2 for instance.

Profiler usage may also introduce signal usage that interferes with simple_pool_manager doing sleep. nanosleep with resume if interrupted should be used there.