rdmd_test: Add --concurrency flag to perform a concurrent stress test
rdmd: Use a lock file to synchronize concurrent builds in the same workDir
rdmd: Fix rebuilding a still-running program on Windows

Windows locks the executable files of running processes.
While the program is running, we can't overwrite or delete
its executable file, but we can rename it out of the way.
Merge pull request #99 from CyberShadow/rdmd-flock

rdmd: Use a lock file to synchronize concurrent builds in the same workDir