CONTRIB: halog: avoid calling time/localtime/mktime for each line

CONTRIB: halog: avoid calling time/localtime/mktime for each line

The last commit provides time-based filtering. Unfortunately, it wastes
90% of the time calling the expensive time()/localtime()/mktime()
functions.

This patch does 3 things :
  - call time()/localtime() only once to initialize the correct
    struct timeinfo ;

  - call mktime() only when the time has changed regardless of
    the current second.

  - manually add the current second to the cached result.

Doing just this is enough to multiply the parsing speed by 8.