Add file list buffer not reduce Gerrit calls

Change-Id: Ice54b495d7ab5d1c08e2bc03004b4a2fa94d8ad4
Split Decorator and PostJob plugins

- ReviewHolder is a singleton used to share ReviewInput between
 GerritDecorator and GerritPostJob
- HashMaps have been changed to ConcurrentHashMap in ReviewInput to handle
 hypothetical concurrent access from GerritDecorator and GerritPostJob

Change-Id: I8f2cadda7b74da91fb784b35bb6279385f2278a4