Partitioning: Use more consistent language regarding partitions and buckets

[RasmusRendal]

In the current state, a partition consists of partitions, which is quite confusing. This PR changes that, such that a partition consists of buckets.

[bruth]

Hi @RasmusRendal, thanks for the contribution. Now that you pointed this discrepancy out, I think removing the term bucket generally would make more sense and stick with partition since it is redundant.

For example:

Suggested change

	For example: new customer orders are published on `neworders.<customer id>`, you can partition those messages over 3 partition numbers (buckets), using the `partition(number of buckets, wildcard token positions...)` function which returns a partition number (between 0 and number of partitions-1) by using the following mapping `"neworders.*" : "neworders.{{wildcard(1)}}.{{partition(3,1)}}"`.
	For example: new customer orders are published on `neworders.<customer id>`, you can spread those messages over 3 partition, using the `partition(number of partitions, wildcard token positions...)` function which returns a partition number (between 0 and number of partitions-1) by using the following mapping `"neworders.*" : "neworders.{{wildcard(1)}}.{{partition(3,1)}}"`.

Does this make sense?

[RasmusRendal]

I think calling the things the partition consists of something different than "partition" is still important, if not for making the documentation understandable, to make it easier to talk about NATS. I want to be able to tell a colleague that "We create a partition of our subject, and each bucket/part is handled by a separate consumer".

This is also how people talk about partitions in other contexts: https://en.wikipedia.org/wiki/Partition_of_a_set#Definition_and_notation

The sets in $P$ are called the blocks, parts, or cells, of the partition.