Avoid usage sleep(1) in perfmarker code

By analyzing WebDAV transfers from DPM it was found suspicious structure in transfer time - transfers finished in 1s intervals. This was caused by sleep(1) function called in the code that sends performance markers every second. To sucessfully finish transfer it was necesary to wait for last performance marker.

This code add new callback that is called immediately once transfer (copy/get/put) is done so it is possible to signal perfmarker loop.

Update also contain fix for small memleak related to perfmarker data structures.