clean up metadata reading
move all methods that talk to db to one area. straight cut+paste
clean up get last string metric value
remove references to metadataserializer serializing non-string values
Revert "stop using a sentinel, just use null"

This reverts commit d4176b3197d7776cfbd0606f5765970111cca4a7.
Merge pull request #197 from fourk/cleanup_reader

Cleanup reader pt1