grandpa: add voting rule to pause new votes for a period (#904)

* grandpa: add voting rule to pause new votes for a period

* grandpa: increase delay

* grandpa: parse custom pause delay from cli

* grandpa: log scheduled pause on startup

* grandpa: rename parameter to grandpa_pause

* grandpa: make pause voting rule generic on block

* grandpa: add test for pause voting rule

* grandpa: add hardcoded pause

* collator: fix test compilation