Fix deadlock due to DateTimeZone.UTC

Referencing DateTimeZone and FixedDateTimeZone from two
threads at startup could deadlock
Add package scoped UTCDateTimeZone class to avoid this

DateTimeZone.UTC has changed implementation type
It used to be a FixedDateTimeZone
Now it is a UTCDateTimeZone
This avoids a potential deadlock scenario
Serialization is unaffected
This change will only be observable if you cast the
UTC constant to FixedDateTimeZone
Fixes #171