(pouchdb/pouchb-server#72) - change PouchDB on the fly

pouchdb/pouchdb-server#72 needs this for two reasons:
1) when the couchdb.database_dir config option changes, it needs to update to the new value without a restart.
2) when starting, it needs to be able to access app.couchConfig for default values for its command line arguments before creating the PouchDB object.