Other ODM support

[zaaack]

This module is awesome, but it has a high coupling with mongoose.

Here are some suggestions:

1. Extract ODM abstract layer to let users write other ODM's adapter.
2. Extract lock abstract layer to using different lock implementation like fs lock, redis lock, etc.
3. Transaction could add a timeout for auto rollback after crash.
4. async/await support
5. ...

[miserylee]

@zaaack All above suggestions will be considered! I'll work on it later after my works in company come to a end. ：）

[zaaack]

@miserylee Thanks!