ctdbd: Set num_clients statistic from ctdb->num_clients

ctdbd: Set num_clients statistic from ctdb->num_clients

This fixes the problem of "ctdb statisticsreset" clearing the number of
clients even when there are active clients.

Values returned in statistics for frozen, recovering, memory_used are based on
the current state of CTDB and are not maintained as statistics.  This should
include num_clients as well.

Currently ctdb->num_clients is unused. So use that to track the number of
clients and fill in statistics field only when requested.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>