s4:rpc_server: use a variable for the max total reassembled request payload

s4:rpc_server: use a variable for the max total reassembled request payload

We still use the same limit of 4 MByte (DCERPC_NCACN_REQUEST_DEFAULT_MAX_SIZE)
by default.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11948

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Jun 23 04:51:16 CEST 2016 on sn-devel-144

(cherry picked from commit 3f36d31c848496bf509db573e4c12821905b448d)