messaging: Do POOL_USAGE via a socket

messaging: Do POOL_USAGE via a socket

This makes debugging run-away processes much more efficient and even
possible at all: If the pool-usage output is more than 256MB, the
previous code could not realloc it and threw it away. Also, it is not
helpful for an already huge process to allocate even more.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Sep 18 21:27:30 UTC 2019 on sn-devel-184