Cherry pick upstream fix for JBOD support

cherry-pick and back ported fixes for JBOD support:
KAFKA-8002; Log dir reassignment stalls if future replica has different segment base offset (apache#6346)

other commit since last release:
Set Bintray-related env variables via repository setting instead of in .travis.yml (#5)
KAFKA-7288; Fix check in SelectorTest to wait for no buffered bytes (apache#6415)