Adding support for mdb as alternative

[Quanah Gibson-Mount]

The OpenLDAP project has for many years used BerkeleyDB as its high 
performance back end database. However, for a number of reasons, it has 
been found to be less than optimal.  In response, the OpenLDAP primary 
developer wrote new database software using an API modeled after BDB's to 
make migration for OpenLDAP and other projects using BDB significantly 
easier.  I was thinking it would be nice to add support for MDB as an 
alternative to BDB in amavis as well.

Some information on testing of MDB with OpenLDAP:

<http://highlandsun.com/hyc/mdb/20111010LDAPCon%20MDB.pdf>
<http://highlandsun.com/hyc/mdb/20111010LDAPCon-MDB.pdf>

--Quanah

-- 
Quanah Gibson-Mount
Principal Software Engineer
Zimbra, Inc
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