The goal of interfaces is to eventually run in their own process,
so we can't use EXCLUSIVE_LOCKS_REQUIRED in their declaration.
However TestBlockValidaty will crash (in its call to ConnectBlock)
if the tip changes from under the proposed block.
Have the testBlockValidity implementation hold the lock instead,
and non-fatally check for this condition.
Set tip at the start of the function and only update it for a long poll.
Additionally have getTipHash return an optional, so the
caller can explicitly check that a tip exists.
