Build fixed size chunks when indexing

Because indexing leaks memory even while queueing we need a way to build
fixed size subprocessesd for reindexing.  This changes the default behavior
of --buildChunks to do just that.  You can still get the old behavior with
--buildChunk 10total.

Also, update documentation.

Bug: 59164
Change-Id: I2babb38c1b1e20c8bf121b83ac0ec8ec35bdd1b6
Shrink the LinksUpdateJob a bit

This'll protect from updates that add or remove a _ton_ of links.

This moves the link selection from after the job starts to before which
means fewer links are saved to the job.  It also stores the links as
prefixed db keys rather than Titles which should make the job itself much
smaller.

Also we can afford to do about 50 links per job and still finish in a
few seconds so change the maximum number of links and unlinks to index
to 25 each.

Bug: 59123

Change-Id: Id0586b798dda57fb69350307e9d1f85b34400a7f
Merge "Shrink the LinksUpdateJob a bit"
Merge "Build fixed size chunks when indexing"