fast-import: introduce mem_pool type

fast-import: introduce mem_pool type

Introduce the mem_pool type which encapsulates all the information necessary to
manage a pool of memory. This change moves the existing variables in
fast-import used to support the global memory pool to use this structure. It
also renames variables that are no longer used by memory pools to reflect their
more scoped usage.

These changes allow for the multiple instances of a memory pool to
exist and be reused outside of fast-import. In a future commit the
mem_pool type will be moved to its own file.

Signed-off-by: Jameson Miller <jamill@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>