MXCACHE-72 - Make mxcache plugin multithreaded

Mark MxCache maven plugin mojo as @threadSafe to prevent warnings during maven parallel build
Increase the threshold for parallel instrumentation (was: 10 files, now: 100 files)
Merge pull request #10 from akochurov/master

MXCACHE-72 - Make mxcache plugin multithreaded