Fixing @Timed method visibility.

Bugs closed: AURORA-966

Reviewed at https://reviews.apache.org/r/28883/
Use pip to pre-fetch python dependencies.

Reviewed at https://reviews.apache.org/r/28915/
Changing the default --batch-size to 1.

Bugs closed: AURORA-961

Reviewed at https://reviews.apache.org/r/28831/
Adding PMD rule to check @Timed annotation placement.

Bugs closed: AURORA-967

Reviewed at https://reviews.apache.org/r/28914/
Adding JMH framework support for scheduler performance analysis.

Reviewed at https://reviews.apache.org/r/28710/
More logging in MaintenanceController.

Bugs closed: AURORA-935

Reviewed at https://reviews.apache.org/r/28806/
Improving logging experience in admin drain_hosts.

Bugs closed: AURORA-943

Reviewed at https://reviews.apache.org/r/28811/
Suppressing redundant client command error messaging.

Bugs closed: AURORA-965

Reviewed at https://reviews.apache.org/r/28876/
Make abstract decorators effective in CommandHook class

Reviewed at https://reviews.apache.org/r/28879/
Changing log level for the RequestLogger.

Bugs closed: AURORA-934

Reviewed at https://reviews.apache.org/r/29005/
Extract mechanism for determining pex version so it can be used for cases other than client version.

Reviewed at https://reviews.apache.org/r/29058/
Make 'pants goal' invocations work.  Use proper python codegen style.

Testing Done:
./pants goal test src/test/python/apache/aurora/client/api:updater

Reviewed at https://reviews.apache.org/r/29126/
get_client_version should never be allowed to run unpatched in test.

get_client_version tries to get the PEX-INFO of sys.argv[0], which is
usually pants when running tests (!) if the format of PEX-INFO changes to a
new version of pants (as is the case for pants 0.0.25), then tests will
start failing with an inscrutible error.  instead, just patch
get_client_version to something sensible.

Reviewed at https://reviews.apache.org/r/29132/
Remove unused 3rdparty dependencies.
Wire up tests not linked to src/test/python:all
Remove unused tests.

Testing Done:
Listed leaf dependencies:

```bash
for target in $(./pants goal list :: 2>/dev/null); do
  if [[ -z "$(./pants goal dependees $target 2>/dev/null)" ]]; then
    echo $target
  fi
done
```

Reviewed at https://reviews.apache.org/r/29267/
Add custom user agent for Aurora v1, Aurora v2 and Aurora Admin clients.

Testing Done:
./pants build src/test/python/apache/aurora::

Also ran e2e tests for v1 and v2 and verified the user agent in the
scheduler logs.

Bugs closed: AURORA-934

Reviewed at https://reviews.apache.org/r/29165/
Fixing build break.

Couple of tests changes escaped in https://reviews.apache.org/r/29165

Testing Done:
./pants src/test/python:all

Reviewed at https://reviews.apache.org/r/29275/
Removing duplicate target.

Duplicate hooks target introduced in https://reviews.apache.org/r/29165

Reviewed at https://reviews.apache.org/r/29348/
Flag to override the address published into ZK
Fixed style issues
Changes from code review feedback