db_mongodb: new module offering DB APIv1 connector for MongoDB NoSQL server

- initial version, tested for INSERT (e.g., can be used with acc module)
- UPDATE and DELETE implemented as well, no option to test them yet
  alone
- SELECT implemented for basic queries, but store result not implemented
  yet -- needs to map the requested fields and types (to be fixed very
  soon)
- no raw query yet (an ndb alternative to follow)