Update the KV REST API. Improved tests, implementations for Contains and Increment. Nested API to support expansion.

Summary:
Add (and fix) tests around checking the HTTP status code for the KV Rest API.

Add HEAD method for /db/entry/KEY which calls to the Contains API. Plus corresponding tests.

Remove stray comment + add useful StatusOK header line (even though framework defaults to the same)

Refactor REST tests for readability and to narrow the surface area of what is being tested.

Send back correct content-type for GET requests. Refactor tests to support content types. Fix test pollution caused by server reuse.

Refactor tests to support different urls in test cases

Add Increment API to REST API.

Test Plan: go test

Reviewers: strangemonad, spencerkimball

Reviewed By: spencerkimball

Subscribers: andybons, team

Differential Revision: http://phabricator.cockroachdb.org/D105