indexing/tdb: More error check in mapistore_indexing_tdb_init and mem context
reordering for easier handling
indexing: Initialize with TALLOC_CTX rather than mapistore_context context

We don't need mapistore_context in indexing at all for indexing to work.
So, brake this not needed dependency and allow indexing to be
more widely used - exposed through Pythong for instance.
mapistore: Expose mapistore_indexing throught mapistore.Indexing object