[PATCH] Implement negative connection cache in terms of gencache

["Marc VanHeyningen"]

Per our discussions of last month, here's a proposed change to implement
the negative connection cache in terms of the gencache.  This permits
all process to share a single cache, rather than each of them
independently having to discover that a server is down.  It amounts to a
complete replacement of libsmb/conncache.c.