Export CalcNextRequiredDifficutly function.

This commit exports a new variant of the existing internal
calcNextRequiredDifficulty function which calculates and returns the next
required difficulty for a block after the end of the current best chain
based on the difficulty retarget rules.

In order to support the exported function the internal one was slightly
modified to accept the block timestamp instead of an entire block since
the timestamp is all that is needed and the caller of the exported
function might next have a full block yet.