*index: --split-shards to speeds initial indexing

*index: --split-shards to speeds initial indexing

When indexing millions of messages, Xapian has a tendency to
slow down as each shard gets bigger.  --split-shards allows
Xapian to work on temporary shards (more akin to "epochs")
and uses xapian-compact(1) to commit the finalized changes
for readers.  The result is roughly 2x faster for millions
of messages.

The downside of this switch is temporary space use increases by
2-3x and incremental changes are not visible to readers until
all indexing is complete.   It has no useful effect on
--reindex, but --reindex is typically faster than initial
indexing anyways since space is already allocated.

It still takes days to create a new extindex of lore, but
fewer days than before.

Another beneficial side effect of this switch is it also
tends to reduce the effect of fragmentation for --cow users
on btrfs.