Windup processor windup 253 rebase

Rebase of https://github.com/windup/windup/pull/228/commits 

    WINDUP-272 Refactor GraphContextFactoryImpl and GraphContextImpl
    WINDUP-253 Refactor WindupProcessor/Impl
    WINDUP-254 Improve error message in RuleSubset.
No, sorry. My mistake! This is correct.