FormXobject should store content stream tokens in the document cache
FormXobject should store content stream tokens in the document cache

* Form XObjects are designed to allow object re-use across pages in a
  document, so there's a high chance they'll need to be tokenised more
  than once when parsing a file
* use the MD5 digest of the form's content stream in the cache key. We
  don't have easy access to the object ID of the form, so this is the
  easiest way to build a unique key
* On my sample document with high Form XObject reuse this commit
  *halved* the time required to extract text from every page

eitoball/pdf-reader,nmadura/pdf-reader,gordonbisnor/pdf-reader,tardate/pdf-reader,yob/pdf-reader,boutil/pdf-reader,nifarius/pdf-reader,yob/pdf-reader,yob/pdf-reader,tdenovan/pdf-reader,alexdowad/pdf-reader