implement remove-before

implement remove-before

The reason to use removeBeforeRemoteEndTime is twofold.

First, removeBefore sends two protocol commands. Currently, the HTTP
protocol runner only supports sending a single command per invocation.

Secondly, the http server gets a monotonic timestamp from the client. So
translating back to a POSIXTime would be annoying.

The timestamp flow with a proxy will be:

- client gets timestamp, which gets the monotonic timestamp from the
  proxied remote via the proxy. The timestamp is currently not
  proxied when there is a single proxy.
- client calls remove-before
- http server calls removeBeforeRemoteEndTime which sends REMOVE-BEFORE
  to the proxied remote.