Update Transaction API getCommittedVersion To Avoid Autoboxing

The `Long getCommittedVersion()` has an overhead of Autoboxing the primitive `long` value to a `Long` object. It's not clear to me the use case for this return value - the JavaDoc do not comment on what a 'null' value returned would indicate.

https://github.com/apple/foundationdb/blob/280b10fa4954e95d18e8ac2f93f520b8822ed2c0/bindings/java/src/main/com/apple/foundationdb/Transaction.java#L244-L254

It looks like the source for this data is primitive already:

https://github.com/apple/foundationdb/blob/280b10fa4954e95d18e8ac2f93f520b8822ed2c0/bindings/java/src/main/com/apple/foundationdb/FDBTransaction.java#L648

Please consider adding a new method with a primitive return type to avoid this overhead.

I happened to come across this when reviewing an application that had churned 9GB of Long values in heap in ~30min - certainly not the end of the world, but something to flag. Thanks.

Start by reading the getCommittedVersion declaration in bindings/java/src/main/com/apple/foundationdb/Transaction.java and the corresponding implementation in bindings/java/src/main/com/apple/foundationdb/FDBTransaction.java. Clarify what a null result means, then determine the Java binding API and coverage changes needed for a primitive-returning alternative; done means the behavior and return-value contract are documented and verified.

Written by the indexing model from the issue text.