disco.erl: remove the unused disco_url_path function
ddfs_gc_node: avoid multiple ets table lookups
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.
disco: add plists as a new dependency for disco

This module will be used to parallelize some of the slow operations.
ddfs_gc_node: traverse through the volumes of ddfs in parallel
erlang scheduler: add the scl scheduler option to the slave nodes
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.
Merge pull request #554 from pooya/ddfs_gc_improvements

some improvements to ddfs gc