approval-distribution: don't fan-out multiple assignments/approvals in separate notifications

[sandreim]

Continued from #732

One idea I have is to have the import_and_circulate_assignment and import_and_circulate_approval functions return the approval/assignment wrapped in an Option. Filtering and everything else should work the same as now. We accumulate these in https://github.com/paritytech/polkadot/blob/master/node/network/approval-distribution/src/lib.rs#L568 and send to peers (this assumes the routing information was updated and is the same for all messages) as a last step.

[alexggh]

Not needed anymore superseded by: #4849