Improve test framework: fork concurrent processes

Add fork(), forkKillAll() and forkWaitAll() framework primitives for running
many concurrent background processes within a single test case.
Add Rhizome Direct stress test

Use new test framework fork() support to run several rhizome direct
push/pull/sync processes at once to a single server.

Speed up setup for Rhizome Transfer stress test.
Fix new Rhizome Direct stress test

Now PASSES with Jeremy's recent fix, but FAILED before