Refactor/remove memoization of index name prefix

[JanBussieck]

There is an open pull request #314 concerning the issue that for multitenant applications the index prefix needs to be set dynamically (#225) which is currently not possible since the index name is memoized. I think there has been some misunderstanding concerning the point of the memoization which is not performance related, but rather a result of the fact that index_name is both a getter and setter (unfortunately) and it should be memoized if set explicitly.

This behavior is preserved in the changes I propose by saving the index_name_stem when it is set while allowing the prefix to be reevaluated every time index_name is accessed.

[pyromaniac]

This code looks really cryptic, the control flow is spread all over the source. I would prefer to have less side-effects in methods.

[JanBussieck]

You are right of course, I moved the memoization into the private method index_name_stem. We'll have the awkward behavior of either setting or computing a value in a single method either way, I moved this down to index_name_stem which is the actual value we want to memoize.

[pyromaniac]

Alright, it looks much better. Could I ask you to add specs to consolidate this behavior? Also I'm going to think about this a little bit more over the weekend.

[JanBussieck]

Sure will do. If you have any other suggestions concerning this change, I'd be happy to incorporate them.

[JanBussieck]

Any idea why this little change causes so much breakage in other places?

[pyromaniac]

Because everything else is counting on the name cache, I'm still going to think how to make it the best way, unfortunately I didn't have a chance yet :(

[pyromaniac]

In master already