Implement Heartbeat statistics output

Default is disabled.

To enable, set environment variable MSTRO_TELEMTRY_HEARTBEAT to
a (decimal, possibly fractional) value of seconds between reports.

Currently will log a CSV line of statistics under INFO priority and 'HB'
thread identifier at every wakeup; once MIO Telemetry is integrated,
the configured telemetry output channels and methods will be used
instead of the logging infrastructure.