Handle FID/MID uniqueness BY MAILBOX rather than BY SERVER

Handle FID/MID uniqueness BY MAILBOX rather than BY SERVER

The FID/MID namespace is unique per mailbox which means that even if
you get out of IDs, you only crash your mailbox, not the entire
server

The following commit introduces a user store object within mapistore.ldb located
under CN=${username},${firstorgdn} responsible from maintaining the GLobalCount
for the user mailbox.

Furthermore, this alters the core behavior of provisioning and introduce username
as being a parameter to most of the mapistore API (propagation scheduled among
mapistore_v1 to mapistore_v2 transition).