V0.31

Merge master to develop

* [adr] Peer behaviour adr updates
* [docs] fix Behaved function signature
* [adr] typo fix in code example

What happened:

New code was supposed to fall back to last height changed when/if it
failed to find validators at checkpoint height (to make release
non-breaking).

But because we did not check if validator set is empty, the fall back
logic was never executed => resulting in LoadValidators returning an
empty validator set for cases where `lastStoredHeight` is checkpoint
height (i.e. almost all heights if the application does not change
validator set often).

How it was found:

one of our users - @sunboshan reported a bug here
#3537 (comment)

* use last height changed in validator set is empty
* add a changelog entry

Fixes #3457

The topic of the issue is that : write a BlockRequest int requestsCh channel will create an timer at the same time that stop the peer 15s later if no block have been received . But pop a BlockRequest from requestsCh and send it out may delay more than 15s later. So that the peer will be stopped for error("send nothing to us").
Extracting requestsCh into its own goroutine can make sure that every BlockRequest been handled timely.

Instead of the requestsCh handling, we should probably pull the didProcessCh handling in a separate go routine since this is the one "starving" the other channel handlers. I believe the way it is right now, we still have issues with high delays in errorsCh handling that might cause sending requests to invalid/ disconnected peers.