Replace local sha1 and md5 implementations with rpm's hash functionality

Replace local sha1 and md5 implementations with rpm's hash functionality
- implement an algorithm agnostic c++ interface to rpm's digest functions,
  based on the former md5/sha1 summation class interface
- minimally "port" md5 + sha1 summation classes on top of the new class
- the result is always used in string format, not bothering to wrap
  MD5/SHA1SumValue class fully, just typedef them as strings and fix
  users