Mongo driver refactoring

Wow, it was much faster and much easier than I have expected. Current pull request status: basic changes, "works for me". Probably some enhancement commit will be added if review will  take some time, otherwise those will go as separate ones.

Changes copied from git log:
```
1) vibe.db.mongo.db -> vibe.db.mongo.client
       MongoDB -> MongoClient
       MongoClient is cleaned up from all database specific features
       MongoClient.opIndex removed
       MongoClient.getDB added
    2) Created new module vibe.db.mongo.db
       Created MongoDatabase class
       All database service functions moved to MongoDatabase
    3) Most vibe.db.mongo.connection stuff is marked as /// private
       All vibe.db.mongo.connection symbols intended for user code usage are documented
    4) getLastError() root implementation is now in MongoConnection
       getLastError() implementation fixed
       getLastError() now return immutable POD struct: MongoErrorDescripion
       checkLastError() now uses getLastError()
    5) MongoHost class -> struct
    6) MongoClient now only uses MongoClientSettings in constructor
    7) MongoCollection stores MongoDatabase instead on plain name string
    8) MongoDatabase basic implementation with old service methods: getLastError, 
    9) MongoDatabase opIndex
getLog, fsync
    9) MongoDatabase.runCommand is package instead of public for better encapsulation

Enhancments:
    1) Added MongoDriverException and MongoDBException
       All mongo driver code now throws one of those
       MongoDBException encapsulated MongoErrorDescripion
    2) MongoCollection.remove() overload added
```