git 29e0d708 broke LastWritten state entry
In Tor 0.2.2.14-alpha, we added two bugs:
A) The LastWritten element of our state became the time of the previous time we wrote the file, rather than the time of this writing
B) When we fail to write the state file, we set LastWritten to -1, which is no good when we hand it to format_iso_time() in read_bandwidth_usage().
This bug was discovered while looking at #2077 (moved).