Adding hash compare of files in RemoteClient.get_file().

 Adding hash compare of file in RemoteClient.get_file(). Avoid downloading if file hasn't changed between local cache and on remote server.
 This doesn't eliminate number of salt calls from minion to master and no great speed up when master and minions are in the same LAN. However it may help, minions over Internet and with slow links.
Merge pull request #6730 from mlister2006/develop

Adding hash compare of files in RemoteClient.get_file().