mapistore/oxio: Implement Indexing using Opechange MySQL database

Hence, _Index class completely refactored into now _Indexing
 * use MySQL mapistore_indexing for Get/Add new URLs
 * use mapistore_indexes to allocate new Index number
   + a conversion into Exchange FMID
 * transparent conertions between Index format for URIs and
   OXIO format of URI

todo:
 * Username should be propagated to the index
 * may be reractor this code as a thin wrapper on top of future
   python bindings for Indexing service