This moves the Status enum out of the BlockUpdate class. This makes it possible to use the same status values for conflicting proofs. Another diff will extract an API from BlockUpdate to make it possilble to build a ProofUpdate class sharing the same interface.

VoteStatus is an enum class and cannot have an operator<<() method, so we need to update the tests to use BOOST_CHECK instead of BOOST_CHECK_EQUAL. Apart from this there is no change in behavior.

Ref T1854.