Cache ordered partitions per event type

[a1exsh]

Move sorting out of the hot path on batch publishing.

Based on findings from #1398

[a1exsh]

~Note that this makes us sort partitions for random-partitioned event types as well, but also only once per publishing batch.~

~We can improve it further by refactoring the PartitionStrategy interface and the PartitionResolver component, but I don't see a clean way to do that for now.~

Now moved the fetching and ordering of partitions all the way up to the event type cache.

[antban]

:+1:

[a1exsh]

:+1: