Fix master arbitration (#198)

* Fix master arbitration

* address comments

* address comments from today's p4rt meeting

* Improve master arbitration text.

* Clarify if there are write requests in flight during master change.

* clarify restarts and other small fixes

* Fix Konstantin's sentence.

* Address Waqar's comments.

* Update docs/v1/P4Runtime-Spec.mdk

Co-Authored-By: Carmelo Cascone <carmelo@ccascone.net>

* Update docs/v1/P4Runtime-Spec.mdk

Co-Authored-By: Carmelo Cascone <carmelo@ccascone.net>

* Update docs/v1/P4Runtime-Spec.mdk

Co-Authored-By: Carmelo Cascone <carmelo@ccascone.net>

* Address comments by ccascone.

* Fix missing election id in the master update rules.

* s/:/. suggested by Carmleo

Co-Authored-By: Carmelo Cascone <carmelo@ccascone.net>