Timing on propfinds; capture long operations

We have had fusedav stats on count and latency of GET and PUT; here we add PROPFIND.
We will also capture operations which take a long time and graph them. The graphs are part of a titan commit. I have chosen 4 seconds for small GET and PUT and 16 for large; 10 for PROPFIND. These are just guesses and can be adjusted as we see how things work out in the wild.