mine: do not submit a solution for a parent that is no longer the tip (#69)

The mining loop tests pindexPrev != pindexBest every 256 hashes, but never
again once a solution turns up. A thread that found one just after somebody
else extended the chain submitted a block built on a stale parent -- a
block born orphaned, at a height already taken.

Every miner thread on a machine works from the same pindexPrev, so the
moment one wins, all the others are hashing a doomed template until their
next checkpoint. On a 32-core box that is thirty threads with a standing
chance of producing a guaranteed orphan.

Re-check the tip inside cs_main before submitting, and drop the solution if
it moved.

Verified on an isolated chain with three miner threads: 2790 blocks
accepted, so nothing legitimate is being discarded, and 2 solutions dropped
for a moved tip -- which is the race actually happening and being caught.

Honest about the size of it: two in 2790 is not what fills a wallet with
orphans. On the 32-core node in production, 1287 of 4117 processed blocks
arrive with a missing parent, which is a different problem and the one
worth chasing next.

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