Cache time conversions for localtime() and CreateTimeString()

When converting a time in seconds (64-bit seconds since 1970) to
Month/Day/Year hours minutes, Suricata calls localtime_r(), which
always aquires a lock and then does complex comutation based on the
current time zone. The time zone can be specified in the TZ
environment variable, which is only parsed the first time it is used,
or from a file. The default file is /etc/localtime. The file is
checked each time to see if it might have changed and is reparsed if
it has changed.

The GLIBC library has a lock inside localtime_r(), which limits
parallelism, which is a problem when the rate of generating alerts is
high, since Suricata generates a new ascii time string for each alert
into fast.log.

This change caches the value returned by localtime_t() and then sets
the seconds within the minute based on the cached start-of-minute
time. All of the values return, expect for the seconds, is constant
within the same minute. Switching to a new seconds could change all
the other values, year, month, day, hour. The cache stores the current
and previous minute values.

The same trick is used in CreateTimeString() for generated time
string. The string, up to the minutes, is cached and then copied into
the result string, followed by printing the new seconds into the
result string.

The seconds within a minute are calculated as the difference in
seconds from the start of the current minute.