Concurrent filepath.Walk replacement

 A concurrent version of filepath.Walk function that
 scans files in a directory tree and runs a callback
 for each file.
 .
 Since scanning (and callback execution) is done from within goroutines,
 this may result in a significant performance boost on multicore systems
 in cases when the bottleneck is the CPU, not the I/O.
 .
 Upstream tests showed ~3.5x average speed increase on an 8-core CPU
 and 8 workers. For measurements, upstream used provided
 bin/traversaltime.go utility that measures directory traversal time
 for both concurrent (cwalk.Walk()) and standard (filepath.Walk()) functions.