HPCC-9827 - Base roxiemem allocator cache on flags as well

Previously the roxiemem cache was based on meta and activity id
only, so it couldn't differentiate between allocators of
different types used by the same allocator.
Also ensure that when Thor activities ask for a unique allocator
they bypass the cache.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>