Move time series metadata to Raft

Fixes #414. All series metadata, that is which series exist, what columns they have, and what the ids of those columns has been moved into Raft.

This means that list series queries are now very fast since they operate on an in memory data structure. It also means that writes no longer have to do gets agains the storage engine to resolve what a column's id is.

Fixes #358. List series now returns as a single series with a `name` column.

This fix has the potential to slow things down in scenarios that people suddenly write many thousands of new series in less than a second. That's because the definition of new series now needs to go through the raft leader and get consensus. I think it's unlikely that it'll be a problem, but we'll need to do extensive testing.

Finally, there is the issue that the Raft snapshots can potentially become big. We should do testing with setups that have millions of series to ensure that performance and startup time remain good.