=pro Upgrade to sbt 0.13.1-RC2 to avoid too many open files.
Merge pull request #1826 from akka/wip-upgrade-to-sbt-0.13.1-RC2-ban

=pro Upgrade to sbt 0.13.1-RC2 to avoid too many open files.
=rem #3706 Avoid loss of floating point precision in PhiAccrualFailureDetector
Merge pull request #1832 from robdavid/wip-3706-avoid-loss-of-floating-point-precision-in-phiaccrualfailuredetector

=rem #3706 Avoid loss of floating point precision in PhiAccrualFailureDe...
!per #3707 Channel enhancements

- Persistent channel
- ConfirmablePersistent message type delivered by channel
- Sender resolution performance improvements
   * unstash() instead of unstashAll()

These enhancements required the following changes

- Unified implementation of processor stash and user stash
- Persistence message plugin API separated from implementation
- Physical deletion of messages
Merge pull request #1827 from krasserm/wip-3707-channel-enhancements

!per #3707 Channel enhancements
!per #3717 Deletion API for snapshots and persistent messages

- single and bulk deletion of messages
- single and bulk deletion of snapshots
- run journal and snapshot store as system actors
- rename physical parameter in delete methods to permanent
- StashSupport.prepend docs and implementation enhancements