Allow individual search backends to control whether they SearchUpdate

$wgDisableSearchUpdate is a sledgehammer for disabling all indexing
which is fine for most people, but sometimes you've got multiple
search backends and you want some to update but not others. This
allows each backend to tell core whether they support the feature.

This also removes the SearchUpdate hook which really doesn't make
sense and nobody uses but CirrusSearch and an abandoned extension
from 7 years ago.

Will require parallel change to MWSearch (Ibd002347) and Cirrus (I87a5b8cc)

Change-Id: I2063fe05000044225fa6bec2171ed0071b84a5cf
Merge "Allow individual search backends to control whether they SearchUpdate"