Reporting of policy errors via host-offline-reason: part 2

Reporting of policy errors via host-offline-reason: part 2

Removing host_status_sender.cc and moving this functionality into
heartbeat_sender.cc.  This avoids multiple issues related to
duplicated code and functionality:
- primarily, this enables sequence-id-based signatures (which are
  better than timestamp based signatures - they don't require the host
  and the bot to have synchronized clocks) without having to maintain
  a separate sequence id for host status messages.
- secondarily, this means we can avoid much code duplication (on the
  client side, but mostly on the server side)

Note that server side currently doesn't handle host-status messages
that are being removed by this changelist.

Handling of host-offline-reason attribute being added by this
changelist needs to be added to server side.  This is being tracked by
cr/78625042

BUG=410050

Review URL: https://codereview.chromium.org/719973002

Cr-Commit-Position: refs/heads/master@{#305756}