When handling fatal signal, shut down Galera networking
before printing out stack trace and writing core file.
This is to achieve fail-silent semantics on crashes which may
keep the process running for a long time, but not fully responding
e.g. due to core dumping or symbol resolving.

Also suppress all Galera/wsrep logging to avoid logging from
background threads to garble crash information from signal handler.

Notice that for fully fail-silent crash, Galera 26.4.19 is needed.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>