Since the TZ backend resorts to ICU for display names, we got
inconsistent results if they disagree about when transitions happened.
Also, ICU uses the current (or only recent history) names for the
zone, so one currently not doing DST (Africa/Tripoli) doesn't get a
report of its DST name at a historical time when it did DST (but ICU
doesn't know about it). Since the ICU backend, in any case, doesn't
override the displayName(qint64, ...), we can simply use the QTZP base
version instead of overloading in the TZ back-end, so we only delegate
to ICU when it might actually help. This also saves duplicating some
locking and lazy-initialization code.
In the process, turn a unique lock into a scoped lock within a
suitable scope. Also, make a comment more grammatical and less
verbose.
Change-Id: Iaaed1fb6a380be8b4258c43cbc4bacd5784345fb
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
1e74f67e54