Faster client lookups

[Alan DeKok]

  I've hacked the "client lookup" code so that it's now looking up
clients in binary trees.  This should allow 10^6 or so clients/server
without the current linear lookup.

  I've updated the SQL "read clients" code, but I don't have SQL, so I
haven't checked if it works.

  The change also removes the "get list of clients" from SNMP feature,
but I don't see that as all that critical.

  Alan DeKok.