record start+end times of hh invocations
Summary:
I'm investigating a rage report. I'm puzzled by the timings shown in the server logs. I'd like to get a timestamp for when `hh rage` invokes two subsidiary invocations of hh (one for server-state, one to get errors). That's what this diff provides.
Other system-wide changes rolled in:
1. I augmented `Process_success.t` and `Process_failure.t` to keep a record of start+end times.
2. I consolidated all "float->timestring" conversions into a single central Utils.timestring function. (previously it existed in Hh_logger and typing_service_state_utils)
Reviewed By: arxanas
Differential Revision:
D21672405
fbshipit-source-id:
f1400aa444c735ba930e182702585cffcaa7917a