oasis-node cmd for wiping the state

[matevz]

Current upgrade process for the node operator requires downloading a new genesis file, wiping the state (but keeping the entity) and run the new oasis node. I suggest we add oasis-node clean command or --rebuild flag or something similar which effectively does (taken from operator docs):

$ rm -rf /serverdir/node/tendermint
$ rm -rf /serverdir/node/bleve-tag-index.bleve.db
$ rm /serverdir/node/abci-mux-state.bolt.db
$ rm /serverdir/node/persistent-store.db

And the next time you run oasis-node it re-downloads and verifies all blocks.

It also seems convenient for node operator support to just say to them "try running oasis-node clean" instead of pointing them to all the rm commands in docs.