Make olson_from_tzstring faster by caching timezone_identifiers_list

In the DAViCal usecase I am trying to optimise (a time-range calquery with expansion), a third (450ms) of wall clock time (1.35s) is spent in timezone_identifiers_list: apparently this is a slow function!

I dropped the conditional on whether timezone_identifiers_list exists, on the assumption that supporting PHP5.1 is no longer a goal :)

Retesting confirms that the request is made ~450ms faster.