scheduler: handle re-scheduling around finalization correctly (#2257)

* scheduler: handle re-scheduling around finalization correctly

* also make sure parathreads get cleaned

* run scheduling in finalization

* Remove stray println!

* Update the schedule call site in inclusion inherent

* Clarify subtlety around SessionStartBlock

* Remove double semi-colon

* reschedule prior to `availability_cores` and in on-initialize

* improve docs

* fix line

* more doc reformat

* remove unneeded call

* avoid unnecessary scheduling on initialize

* split `clear` and `schedule

* Update runtime/parachains/src/scheduler.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

Co-authored-by: Sergei Shulepov <sergei@parity.io>
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