Add a flag to deduplicate storage snapshots

Add a new format for deduplicated storage snapshots. Microbenchmarks
show a 10x deduplication ratio on Twitter's production snapshots.

This format is backwards-incompatible, so this patch introduces a
flag to control its use (defaulting off).

This only changes the format used to write to the replicated log (where
time is of the essence since all writes are done holding the global
storage lock) - the format of backups written to disk is unchanged,
as backups don't hold the lock.

Testing Done:
./gradlew -Pq build

Bugs closed: AURORA-722

Reviewed at https://reviews.apache.org/r/26478/
Remove use of the getVersion RPC from the client.

Bugs closed: AURORA-142

Reviewed at https://reviews.apache.org/r/26714/
Fix errors in help rendering:

- Put plugin-generated options into the correct order.
- Include the option-name in the detailed help list.
- Add missing metavars.

Bugs closed: aurora-831

Reviewed at https://reviews.apache.org/r/26688/
Add a metric to count offers that do not provide sufficent resources
to launch a GC Executor

Testing Done:
./gradlew -Pq build

Bugs closed: AURORA-843

Reviewed at https://reviews.apache.org/r/26809/
Remove deprecated configuration options.

Bugs closed: AURORA-333

Reviewed at https://reviews.apache.org/r/26430/
Handle resourceOffers callback asynchronsly.

Bugs closed: AURORA-848

Reviewed at https://reviews.apache.org/r/26821/
Fix redirect loop when accessing rewritten paths.

Testing Done:
./gradlew build -Pq

Also started up multiple schedulers in vagrant image and confirmed
that I'm properly redirected when accessing the non-leading scheduler.

Bugs closed: AURORA-841

Reviewed at https://reviews.apache.org/r/26790/
Re-add build message pointing to code coverage report.

Reviewed at https://reviews.apache.org/r/26787/
Remove deprecated thermos README.md.

Bugs closed: AURORA-61

Reviewed at https://reviews.apache.org/r/26837/
Cron documentation.

Testing Done:
Pushed to Github.

Bugs closed: AURORA-440

Reviewed at https://reviews.apache.org/r/26836/
Remove LogModule.

Reviewed at https://reviews.apache.org/r/26849/
Adding  a high level storage architecture write up.

Bugs closed: AURORA-839

Reviewed at https://reviews.apache.org/r/26845/
Deprecating SANDBOX_DELETED task state.

Bugs closed: AURORA-751

Reviewed at https://reviews.apache.org/r/26664/
Make README.md more informative.

Bugs closed: AURORA-828

Reviewed at https://reviews.apache.org/r/26830/
Fix table of contents in configuration-tutorial.md

Bugs closed: AURORA-844

Reviewed at https://reviews.apache.org/r/26822/
Describe how to clone the git repo to contributing docs.

Bugs closed: AURORA-504

Reviewed at https://reviews.apache.org/r/26841/
Adding executor health check disabler

The health check disabler allows health checks for a job
to be snoozed temporarily by touching a snooze file in
the job's sandbox.

Bugs closed: AURORA-795

Reviewed at https://reviews.apache.org/r/26383/
Adding wait loop into host_drain status monitoring.

Bugs closed: AURORA-820

Reviewed at https://reviews.apache.org/r/26458/
Simplify the ResourceSlot class.
Hacky Wiring.