[mms] Add Horde_Mongo_Collection_Index interface.
[mms] Add Horde_Mongo_Client#checkIndices() and Horde_Mongo_Client#createIndices().
Move index generation into interface method
Add index to IMP_Sentmail MongoDB.
NoSQL indices related improvements.

[mms] Add the Horde_Core_Nosql library.
[mms] Add the Horde_Registry_Application#nosqlDrivers() method.
[mms] Add ability to update/create NoSQL indices from the admin config page.
Allow various storage drivers to be accessed directly
Return list of NoSQL drivers for the base Horde application

Dealing with MongoDB drivers this way is a bit hackish.  But we can't
take a "migration" approach like SQL databases for various reasons (not
to mention that the migration approach - at least the CLI method - is
pretty much worthless for anything other than the base SQL config set in
conf.php).

But this approach works and is clean from a UI perspective, so it
can/should be used for Horde 5.1.
Implement indices for MongoDb prefs DB
Implement indices for MongoDb sessionhandler DB