Clean up idx/memory namespace by moving components into their own packages

[replay]

We currently have a lot of independent components mixed together in the idx/memory package. This leads to quite a lot of unnecessary namespace pollution in the index package. It should be simple to move many of these components into their own packages.
There could be a hierarchy like

• idx/memory/tags
• idx/memory/tags/metaTags
• idx/memory/tags/metaTags/enricher
• idx/memory/tags/metaTags/metaRecords
• idx/memory/tags/metaTags/metaTagIndex

[Dieterbe]

good luck 😎

[Dieterbe]

looking at https://godoc.org/github.com/grafana/metrictank/idx/memory , it actually looks quite reasonable to me. but this only shows public types.
grepping in idx/memory, i see 28 types declared (public+private)
looking at https://godoc.org/net/http for inspiration, I note that they also have many things related to http together in one package (e.g. servemux, client, request, response, ...)
net/http has 48 .go files, idx/memory has only 20.

Perhaps we could reorganize the code a bit, but i don't see a big problem here.

[Dieterbe]

related: #1510

[replay]

If we don't break it up into small sub-packages, we should at least re-organize the tests a bit because at the moment they're kind of messy and all mixed together. I think this could be improved by moving them into specific files where each file has a defined purpose.

[stale]

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