[HBASE-8185] Pulling HRegionScanner code outside the HRegion class.

Author: manukranthk

Summary: This diff is to decompose the diff D708638 into few smaller diffs for a quicker and easier review process.

Test Plan: Since this is a refactoring change, testing the code change by running MR Unit tests.

Reviewers: liyintang, rshroff, aaiyer

Reviewed By: liyintang

CC: hbase-eng@, erling, arice

Differential Revision: https://phabricator.fb.com/D743249

Task ID: 2103689

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1460199 13f79535-47bb-0310-9956-ffa450edef68