* blocksync: wait for poolRoutine to stop in (*Reactor).OnStop

blocksync.(*Reactor).poolRoutine goroutine lifetime was not supervised,
so OnStop returning did not provide a guarantee that it was done.  As a
result, the other services could be stopped and a subsequent call to
bcR.blockExec.ApplyBlock would cause a panic, typically a leveldb
closed error.

This change uses a sync.WaitGroup to ensure that OnStop returns after
poolRoutine has returned.

This also triggers the poolRoutine method to return on a signal from
bcR.pool.Quit() instead of just bcR.Quit(), which seems to be important
as (*Reactor).OnStop itself can only stop the BlockPool (bcR.pool), while
the BaseReactor.BaseService's quit channel will only be closed _after_
(*Reactor).OnStop has returned.

* rename wg field, comment on strayish select before retry

* break poolRoutine on either Quit before quick retry

* update changelog

* break the loop with an IsRunning check

* do the waitgroup Add and Defer at the same site for clarity

* check both services IsRunning in poolRoutine

* fix oopsie

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Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>
(cherry picked from commit 7ea352e)