(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.

Ah, okay. In that case yeah, it's better to give the user a useful error.