Ensure orhpaned series cleaned up with shard drop

[e-dard]

This PR fixes an issue where series could be left in the inmem index, when they should have been removed.

It could be triggered as follows:

• insert some series into a shard that only exist in that shard;
• wait for the retention policy enforcement to remove the shard;
• run a SHOW query such as SHOW TAG KEYS;
• The series that were removed with the shard will be missing (as expected);
• re-insert the same series;
• run a SHOW query such as SHOW TAG KEYS;
• The series will not be present (they should bed).

This manifests itself because the series were removed from the series file, but not from the inmem index. If the server was restarted the issue would resolve itself.

Commit f52de2d ensures that the series are removed from the inmem index if they're the last occurence of those series in the index.

TODO: whilst doing this work, it's become apparent that dropping shards with the inmem index is particularly racy, when one considers concurrent writes of the same series. I will add a ticket to track that.

[jacobmarble]

Thanks for breaking this up into multiple commits.

[stuartcarnie]

LGTM