[Cache][Lock][Messenger] Track the Symfony component in the MongoDB driver handshake

Report which Symfony component opened a MongoDB connection, so that the
server logs and $currentOp tell them apart:

 * symfony-mongodb-cache, renamed from the cryptic sfcache
 * symfony-mongodb-lock, when MongoDbStore builds the manager itself
 * symfony-mongodb-messenger, when the transport builds the client itself

A driver name set by the application is kept and appended to, so an
application that already identifies itself is not overwritten.

Also fix the tab indentation left on the default_mongodb_provider line,
and replace uniqid() with random_bytes() in the MongoDB transport, the
last uniqid() call in the source tree.