ddfs_gc: introduce the foreach_file function

It goes through all of the files and calls the call-back function for all of
them (just like the lists:foreach) without capturing their return value.

Conflicts:
	master/src/ddfs/ddfs_util.erl
backport the plists dependency.
ddfs_gc_node: traverse through the volumes of ddfs in parallel

Conflicts:
	master/src/ddfs/ddfs_gc_node.erl
ddfs_gc_node: avoid decreasing the priority of the gc process on slaves

Although this process is a background process, it is important for it to
finish in a timely manner.
avoid scheduler compaction on the slave nodes.

We cannot use stbt because there are other processes on the slaves.