* Tally and duration counters for Job Queue tasks and RPC calls
optionally rendered by server_info and server_state, and
optionally printed to a distinct log file.
- Tally each Job Queue task as it is queued, starts, and
finishes running. Track total duration queued and running.
- Tally each RPC call as it starts and either finishes
successfully or throws an exception. Track total running
duration for each.
* Track currently executing Job Queue tasks and RPC methods
along with durations.
* Json-formatted performance log file written by a dedicated
thread, for above-described data.
* New optional parameter, "counters", for server_info and
server_state. If set, render Job Queue and RPC call counters
as well as currently executing tasks.
* New configuration section, "[perf]", to optionally control
performance logging to a file.
* Support optional sub-second periods when rendering human-readable
time points.
