Add a proxier that does what the bouncer does, but proxies requests instead of bouncing them.

The current bouncer tests are more end-to-end tests. We should keep these but also add tests for the individual bouncer request handler methods.

See #1 for the initial bouncer work.
Actually, the default cleanup implementation doesn't work very well for our needs. It isn't included in the timeout handling (which means it can block forever) and my attempts to use it for `MockHttpServer` shutdown in praekelt/go-api-toolkit failed for reasons I wasn't able to discover.