Add support for large object creation in replication mode.

Add support for large object creation in replication mode.
We rewrite lo_creat function call to lo_create which allow us to
specifiy large object id. The missing argument for lo_create is taken
by:
SELECT coalesce(max(loid)::INTEGER, 0)+1 FROM
pg_catalog.pg_largeobject;
To avoid race condion, we add new directive called lobj_lock_table
which is used to aquire lock while grabing above value.
New source file pool_lobj.c added.