Improve MongoDB Session Adapter

- improve performance: prevent multiple findOne() on read() and write()
- improve performance: do not write, if session data did not change
- improve performance: use session_id() as _id to use index
- implement garbage collection
- fix doc block
- streamline variable names
Implement \SessionHandlerInterface
Do not implement \SessionHandlerInterface to keep compatibility with PHP 5.3
Merge pull request #149 from fabfuel/master

Improve MongoDB Session Adapter