FLUME-2502. Improve Spool Directory Source's performance by not listing files each time.

(Prateek Rungta via Hari)
FLUME-2499. Include Kafka Message Key in Event Headers.

(Ricky Saltzer via Hari)
FLUME-2486. TestExecSource fails on some environments

(Santiago M. Mola via Hari)
Merge branch 'flume-1.6' into kafka-channel-uuid-consumer