As part of TS-2362 (Cache backwards compatibility) I must be able to switch the URL hashing at run time. Some release after 3.2 changed the URL from MMH to MD5 so it must be switched back to be compatible but not if there are no old cache stripes. There has also been a pending desire for other reasons by other people for making this more dynamic. The assumption that the crypto hash result is 128 bits is maintained.
This also does a lot of cleanup on use of MD5 hashing. In particular the standard sequence of init context, hash, finalize, has been moved in to a class (URLHashContext) rather than being explicit every time. I have a patch that I am currently testing and will attach it to the bug later today. I'm breaking TS-2362 in to some smaller commits due to feedback from my last large update :-).
