cli: Make PollJob generic to support other protocols

By separating the LUXI-specific code and stdio-related code
into separate classes, we can make cli.PollJob protocol-
agnostic, allowing it to be used with RAPI.

This patch also adds unittests for cli.PollJob.

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