Meta tag support bigtable

[replay]

This implements storing meta records in a new bigtable meta record index. Additionally it refactors the cassandra index so that the meta record index is now also split from the rest of the cassandra index in it's own pkg. It moves some of the config parameters for the cassandra backed meta record index around, so if this code change gets accepted then i'll update the breaking changes log accordingly.

[Dieterbe]

It moves some of the config parameters for the cassandra backed meta record index around,

this seems weird. especially because so many settings that are normally shared (hosts, keyspace, auth, timeout, ...) are now duplicated. Why would anyone want to store half of their index data in one cassandra cluster and the other half in a different cluster (or keyspace)? This seems very unlikely and unnecessarily complicated.
It also seems annoying to update settings in 2 places and having to keep them in sync. and can lead to trouble if a mistake is made in this process.

[replay]

The idea is to split the whole meta tag index away from the current main index and into it's pkg. I could just copy the config settings of the main index into the meta tag index as well, but then why did we split the store and the index into separate config sections? Isn't the argument there to just provide as much flexibility as possible?

[robert-milan]

Just a few things to start

[replay]

I rebased this PR and went over all the changes one more time to make sure everything is ok, and to familiarize myself with it again.

I didn't make any large changes, @robert-milan you should be able to see the diff of the changes I made today by doing git diff 41db82 e3df92 after fetching.

Out of all the changes I made, the largest one was in store/bigtable/bigtable.go because there I'm now using the EnsureTableExists() utility from bigtable/util.go, which previously wasn't used there.

If you're still ok with these recent changes, then I'll go ahead and merge.

[robert-milan]

I rebased this PR and went over all the changes one more time to make sure everything is ok, and to familiarize myself with it again.

I didn't make any large changes, @robert-milan you should be able to see the diff of the changes I made today by doing git diff 41db82 e3df92 after fetching.

Out of all the changes I made, the largest one was in store/bigtable/bigtable.go because there I'm now using the EnsureTableExists() utility from bigtable/util.go, which previously wasn't used there.

If you're still ok with these recent changes, then I'll go ahead and merge.

Sure, go ahead