[master] Customized Executor for HTableClientScanner

Author: daviddeng

Summary:
`HTableClientScanner` defaultly use `HTable.multiActionThreadPool`
A `Builder` is defined for `HTableClientScanner`
The user can specify an alternative `Executor` in `Builder`

Test Plan: `TestHTableClientScanner`

Reviewers: manukranthk, liyintang, fan

Reviewed By: manukranthk

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1287866

Task ID: 4171349, 3546998

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1589234 13f79535-47bb-0310-9956-ffa450edef68