Allow setting default schema using REST API / Web UI

Currently, to set the default schema you must run the `USE <schema>` command.  However, the Web UI and REST API do not keep a session open so the `USE` command does not affect the next query sent to the API.

To support a default schema for REST API & Web UI I propose a parameter to the API "defaultSchema" which sets the default schema for that query.

Example: curl -d '

{"query":"SHOW FILES","defaultSchema":"dfs.tmp","queryType":"SQL"}

' -H 'Content-Type: application/json' -H 'User-Name: test' localhost:8047/query.json