s4:rpc_server/netlogon: keep a global challenge table

s4:rpc_server/netlogon: keep a global challenge table

Some clients call netr_ServerReqChallenge() and netr_ServerAuthenticate3()
on different connections. This works against Windows DCs as they
have a global challenge table.

A VMware provisioning task for Windows VMs seemy to rely on this behavior.

As a fallback we're storing the challenge in a global memcache with a fixed
size. This should allow these strange clients to work against a
Samba AD DC.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10723

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>