Fix constraint violation caused by slave ID change.

Bugs closed: AURORA-596

Reviewed at https://reviews.apache.org/r/23832/
Implementing parallel updater.

The updater now spawns upto batch_size threads to process
one instance per thread.

All scheduler calls are multiplexed by the SchedulerMux to do
batch kill/add/restart/status calls.

Bugs closed: AURORA-350

Reviewed at https://reviews.apache.org/r/21440/
Filtering out unknown hosts when starting maintenance.

Bugs closed: AURORA-443

Reviewed at https://reviews.apache.org/r/23827/
Database-backed implementation of SchedulerStore.

Bugs closed: AURORA-555

Reviewed at https://reviews.apache.org/r/23107/
Wire up announcer into thermos executor.
When syncing code to vagrant, delete files that were deleted from the repository.

Reviewed at https://reviews.apache.org/r/23859/
Rework to make more compatible with diversity of Kazoo options.
Update vagrant image to enable announcing.
Enable announcer in end to end test.
AURORA-594: RC1 files missing Apache license headers

Adds missing license headers to empty and newly added files.

Testing Done:
ran: build-support/jenkins/build.sh

All test passed except test_cluster_option as outlined in AURORA-595

Bugs closed: AURORA-594

Reviewed at https://reviews.apache.org/r/23718/
AURORA-590: Release scripts branch and CHANGELOG urls are wrong in generated email template

Fixes urls in email templates to point to correct locations

Testing Done:
The CHANGELOG for the release is available at:
https://git-wip-us.apache.org/repos/asf?p=incubator-aurora.git&f=CHANGELOG&hb=0.5.0-rc1

The branch used to create the release candidate is:
https://git-wip-us.apache.org/repos/asf?p=incubator-aurora.git&hb=0.5.0-rc1

Bugs closed: AURORA-590

Reviewed at https://reviews.apache.org/r/23688/
AURORA-599 - Reset changelog and version for 0.5.0 rc2

Resets the CHANGELOG and .auroraversion back to state it was
before 0.5.0-rc1 was cut. Jira fixVersion fields have been updated
as well to match any recent resolved tickets since 0.5.0-rc1
was cut that did not get tagged correctly.

Bugs closed: AURORA-599

Reviewed at https://reviews.apache.org/r/23876/
Fix problem with deschedule command.

Bugs closed: aurora-597

Reviewed at https://reviews.apache.org/r/23872/
Updating CHANGELOG for 0.5.0 release.
Incrementing snapshot version to 0.5.1-SNAPSHOT.
Add test case to end-to-end test to validate that a serverset has been announced.  Add headers to announcer files.
Add executable bits back to end to end tests.
Address Kevin's comments
Refactoring SchedulerCore (forceTaskState)

Bugs closed: AURORA-94

Reviewed at https://reviews.apache.org/r/23835/
Refactoring SchedulerCore (restartShards)

Bugs closed: AURORA-94

Reviewed at https://reviews.apache.org/r/23834/