Logging fast things

Logging fast things

In many cases HITs logged with zero response times. The logging entries are
correct, those transaction took less than a millisecond. However, to better
monitor Squid performance and to optimize things further, a user may want to
see more precise response time measurements logged.

Squid already computes response times with microsecond resolution
(timeval::tv_usec), which would be enough for any modern measurement, but
Squid loses that precision due to tvSubMs conversion.

This patch add configurable "precision" for time-related fields such as %tr,
using a .n syntax similar to the syntax used by the well-known printf(3) API
to mean "maximum field width".

Examples:
  %tr   -- 0 -- millisecond precision (no change compared to today)
  %.3tr -- 0.123 -- microsecond precision (after this project)
  %.6tr -- 0.123456 -- nanosecond precision (year 2050 trading platform??)

This is a Measurement Factory project