allow multiple slaves on the same node
allow multiple slaves on the same node

This makes the slaves use file locking to exclusively own their
cache directory. If the directory is already locked, they use
a suffix to differentiate their own. This works around the fact
that run_isolate.py isn't safe to run concurrently on the same
cache directory.

It's wasteful in terms of cache capacity, but this is an easier
fix than fixing isolate itself.

umbrant/dist_test,xiao-chen/dist_test,xiao-chen/dist_test,cloudera/dist_test,umbrant/dist_test,umbrant/dist_test,cloudera/dist_test,xiao-chen/dist_test,cloudera/dist_test,umbrant/dist_test,cloudera/dist_test,xiao-chen/dist_test