BMessageFormat: parse the pattern at construction

* Instead of parsing the pattern everytime Format() is called, parse it
only once when the object is created.
* Adjust all callers to make use of the feature and reuse the instance
as much as possible. This also allows calling B_TRANSLATE only once
instead of everytime the formatting needs to be done. We use either a
static instance (when the message pattern is constant) or a field (when
it is not known to be constant).
* Since the BMessageFormat instances are now reused, add locking to
avoid race conditions (ICU itself is thread safe, but the format pattern
is recreated when the locale is changed)