datetime: use Olson for timezone name

* Modified localtime.c to use tnt_tm instead of tm structure;
* Use this structure for passing data to tnt_localtime_rz at
  the moment of time-zone resolution;
* As a side-effect of refactoring - simplified singleton we
  used to use for automagical sorting of array with timezone
  abbreviations

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(cherry picked from commit 2116bbfd)