Updates to the way meta indexing is handled for filestore. (#4450)

Historically we kept indexing information, either by sequence or by
subject, as a per msg block operation. These were the "*.idx" and
"*.fss" indexing files. When streams became very large this could have
an impact on recovery time. Also, for encryption the fast path for
determining if the indexing was current would require loading and
decrypting the complete block.

This design moves to a more traditional WAL and snapshot approach. The
snapshots for the complete stream, including summary information, global
per subject information maps (PSIM) and per msg block details including
summary and dmap, are processed asynchronously. The snapshot includes
the msg block and has for the last record hash that was considered in
the snapshot. On recovery the snapshot is read and processed and any
additional records past the point of the snapshot itself are processed.
To this end, any non-system removal of a message has to be expressed as
a delete tombstone that is always added the the fs.lmb file. These are
processed on recovery and our indexing layer knows to skip them.

Changing to this method drastically improves startup and recovery times,
and has simplified the code. Some normal performance benefits have been
seen as well.

Signed-off-by: Derek Collison <derek@nats.io>