DifferenceEngine: Improve cache invalidation

Invalidate the diff cache if the engine producing the diff changes, or
if a configuration setting that controls the diff output changes. This
is probably what most users expect, that changing the configuration will
result in a change for diffs that may have already been viewed.

For wikidiff2 specifically, a change in version or
$wgWikiDiff2MovedParagraphDetectionCutoff will invalidate the cache.

Refactor engine detection and sanity-checking into a private getEngine()
function.

As part of this getDiffBodyCacheKey() was deprecated, and subclasses
should implement getDiffBodyCacheKeyParams() instead. Drop the
deprecated and unused MW_DIFF_VERSION constant while we're at it, and
bump DIFF_VERSION since we're already changing the cache key format.

Bug: T180043
Change-Id: I4e386ca05bd2a2fb54208d760c131eb42e3a72ab