Loading the last 3 successful builds may be very expensive in certain rare, but possible situations (all newer builds failed, only some very old builds were successful).
Go only up to 6 build into the past for calculating the estimated duration.
Also take failed builds into account, if we don't find any successful ones.
fixed typo
reduce visibility of getEstimatedDurationCandidates to protected
Merge pull request #837 from kutzi/estimated-time

[FIXED JENKINS-18196] Change to the estimated duratrion calculation to prevent performance issues with lazy build loading