Replace files in stats/ rather than appending to them.
Right now, we append statistics to files in the stats/ directory for half of the statistics, whereas we overwrite these files for the other half. In particular, we append buffer, dirreq, and entry stats and overwrite exit, connection, and bridge stats. Appending to files was useful when we didn't include stats in extra-info descriptors, because otherwise we'd have to copy them away to prevent Tor from overwriting them. But now that we include statistics in extra-info descriptors, it makes no sense to keep the old statistics forever. We should change the behavior to overwriting instead of appending for all statistics. Implements #2930.