This adds various new locks to replace the single big lock on
the head. All parts now must be COW as they may be held by clients
after initial retrieval.
Series by ID and hashes are now held in a stripe lock to reduce
contention and total holding time during GC. This should reduce
starvation of readers.
This adds interval metadata to indexed chunks. The queried interval
is used to filter chunks when queried from the index to save
unnecessary accesses of the chunks file.
This is especially relevant for series that come and go often and larger
files.
