Improve query coordinator timeouts.

- add a query timeout, set to 2m by default for now.  Previously a bug between
socrata-http and apache commons httpclient was causing the get-schema-timeout to be
used for the query.

- Switch to actually use connect-timeout value, and not just get-schema-timeout
twice.

- Respond with a HTTP 504 Gateway Timeout when we have a timeout talking to the secondary.
This currently just gets translated by soda fountain to a generic 500.