backend: Implement remote commands

As per design document (doc/design-remote-commands.rst), a number of
rather strict tests is applied to any incoming request, a delay is
inserted upon errors and returned error messages are very generic
(unless it's the actual command that failed). There are unit tests for
all of the newly added code.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>