[master] Make TestHTableMultiPutThreadPool more stable

Author: daviddeng

Summary:
Since `ThreadPoolExecutor.getCompletedTaskCount()` is not very accurate, sometimes the task may be finished but the count is not increased because the thread has been switched.

Add a sleep before read the number.

Use `AssignmentLoadBalancer` to make the creating of table more stable.

Test Plan: `TestHTable`

Reviewers: manukranthk, gauravm, liyintang, rshroff, adela, elliot

Reviewed By: adela

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1305847

Task ID: 3296253

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1591994 13f79535-47bb-0310-9956-ffa450edef68
[master] Add LongAggregator endpoint.

Author: daviddeng

Summary:
Implements min/max/sum fuctions.
Some refactoring.

Test Plan: `TestLongAggregator`

Reviewers: adela, manukranthk, gauravm

Reviewed By: adela

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1297685

Task ID: 4223215

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1591995 13f79535-47bb-0310-9956-ffa450edef68
[master] Make TestDrainRegionServer stable.

Author: elliott

Summary: Make sure TestDrainRegionServer is stable by waiting for all regions to settle.

Test Plan: run TestDrainRegionServer 25x locally.

Reviewers: daviddeng, aaiyer

Reviewed By: daviddeng

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1307493

Task ID: 4179922

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1591996 13f79535-47bb-0310-9956-ffa450edef68