mapistore/Indexing: Convert allocated FMIDs to exchange format

Pretty much done everywhere and it seems this is what Exchange expects
mapistore/oxio: Implement _Indexing wrapper arround mapistore.Indexing interface

Added value is that _Indexing class makes sure that all URL
are in oxio:// namespace and converts automatically URLs in OXIO format
mapistore_test: Do not hardcode initial Context FMID but peek into Indexing first

While harcodding the FMID works well with in-memory mapistore python
adapter, it doesn't fit well with OXIO adapters since OXIO
works with real URLs both from OX site and indexing
mapistore/oxio: Make use of new indexing interface implementation
mapistore/oxio: Log Folder IDs as decimals - easier to map to what we have in DB
pymapistore: Fix MAPIStore.initialize() to pass proper mstore_ctx
mapistore/python: Convert PT_I8 using PyLong_AsLongLong()

PyLong_AsUnsignedLongLong() fails with large numbers with value
overflow, which leads to *very* hard to track exception
mapistore/oxio: Implement list_contexts() to return INBOX context desription
mapistore/oxio: Do not index parent folders above the ROOT folder we support
mapistore/oxio: Use python logger for tracing rather than plain print

Make debugging really possible