bankd: Avoid osmocom logging mutex deadlock in signal handling
The main thread communicates slotmap add + delete via POSIX signals to the worker threads. As those signals interrupt the normal processing of the worker thread, they might get delivered while the thread is already logging something, causing a deadlock. This has been observed in the real world in the following stack trace (where it's actually two nested signals):
As a hot-fix, let's avoid logging from the handle_sig_map{del,add}() functions at all, making them safe against a deadlock around this mutex.
We should decide how to proceed in general with potentially some architectural changes later on; any such changes are not suitable as a hot fix due to their potential of introducing other regressions.
