FixedDateFormat improvements

Proposed changes:

• improve thread-safety of FixedDateFormat::updateMidnightMillis
• expose FixedDateFormat::millisSinceMidnight as a public method

In multi-threaded scenarios where time does not move forward monotonically, the implementation of FixedDateFormat::updateMidnightMillis can result in corrupted time stamps. In a project I am involved in we have a custom PatternConverter that uses FixedDateFormat to format the "event time". In these (artificial) tests, event time does not always move forward, so the updateMidnightMillis is called concurrently with varying values. This is not a production issue, but the implementation can be improved to be thread-safe without impacting performance by using double-checked locking.

Making FixedDateFormat::millisSinceMidnight public would provide a performant and convenient way to strip off the date component. Useful for systems that are on Java 7 or for systems on Java 8 that don't want to construct a LocalTime object every time this value is required.