move releasedate finding code
remove need to lookup sha
simplify ReleaseDateFinder
split out LastVersionOnMasterFinder
push sha and date up

so they are in the build metadata and not the release date
remove OriginalRelease

fixes #266
fixes #267
move cache to task

no need for in memory cache in an exe
use LastVersionOnMasterFinder in task
make LastVersionOnMasterFinder return a date
remove concept of releasedate from the core
add githubflow todo
better use of when
fix some error
simplify OptionallyTaggedBranchVersionFinderBase
remove OptionallyTaggedBranchVersionFinderBase

fixes #214