Consolidate common Preprocessor caching code

Consolidate common Preprocessor caching code

* Consolidate nearly-identical caching code in Preprocessor_DOM and
  Preprocessor_Hash by making Preprocessor an abstract class rather than an
  interface and by implementing Preprocessor::cacheSetTree() and
  Preprocessor::cacheGetTree().
* Cache trees for wikitext blobs that have length equal or greater to
  PreprocessorCacheThreshold. Previously they needed to be greater than
  PreprocessorCacheThreshold, so this changes the requirement by one character.
  I did it because it seems more natural.
* Modernize the code to use singleton service objects rather than globals.

We spend a lot of time in the Preprocessor, so it would be nice for this code
to be well-factored and clear.

Change-Id: Ib71c29f14a28445a505e12c774a24ad964330b95