fix regression on openchangedb ldb backend related with public folders

Folder ids for public folders must be obtained in a special way when
using mysql backend, but if we are using ldb backend we have to obtain
them like always (using mapistore_indexing_*).

openchangedb_is_public_folder_id will return always false when we
are using ldb backend and will return the proper value (depending of the
folder id given as parameter) with mysql backend.
refactor get new public folder id into function
retval checks on emsmdbp_mailbox_provision_public_freebusy
Merge pull request #89 from Zentyal/jgarcia/fix-ldb-public-folder-ids

Jgarcia/fix ldb public folder ids