started the expiry implementation for checkpoints
merged with master and finished the integration
added a missing passed parameter for expiry
Merge branch 'master' into checkpoint-expiry
fixed the archiving policies of checkpoints
setting the number of reducers to 0 when there is no gbk and using the output channels to output values
updated specs2
using FiniteDuration on checkpoint expiryTime and added some doc
fixed the failing unit test