Implement _purge API

[lokingwei]

In our production deployment, there's document with 1k of conflict will cause CouchDB cluster crash in fabric layer

As suggested by others, we need to constantly purge CouchDB conflict docs (including deleted) to keep up with the performance.

Reference
https://stackoverflow.com/a/50548875

We need to implement _purge in foodb client so that we can perform the DB optimisation on client side