[HBASE-6967][89-fb] Adding a way to dynamically add a java class into the classpath for specifying compaction hooks

Author: adela

Summary:
User can specify a jar and a class which will be loaded from
the jar; it should be specified via alter table command, example:
alter 't3', {NAME => 'f1', CONFIG => {'compaction_hook_jar' => '/home/adela/local/latest_jar/simplehook.jar', 'compaction_hook' => 'org.apache.hadoop.hbase.regionserver.compactionhook.SimpleTestCompactionHook'} }

Test Plan: tested with compaction hooks

Reviewers: liyintang, kannan

Reviewed By: liyintang

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D680766

Task ID: 2027229

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1434869 13f79535-47bb-0310-9956-ffa450edef68