mine: ask whether a transaction fits before appending it

The node miner tested its size budget only on the outer while loop. The inner
for walked all of mapTransactions with no test, so a single pass could append
the entire mempool -- the overshoot was bounded by mempool size, not by one
transaction as the comment claimed.

The pool builder did test, but only after the push, which bounds the overshoot
at one transaction. Nothing caps how big one transaction may be: neither
CheckTransaction nor AcceptTransaction limits it, so the mempool will hold one
of any size the wire allows and "one transaction" is worth up to MAX_SIZE. A
lone 1.5 MB transaction offering a large fee is therefore enough to make every
template unmineable, and the fee is never paid because the block never
confirms.

Both now ask before appending and skip what does not fit, so the overshoot is
zero and an oversized transaction costs the template nothing rather than
invalidating it. The budget moves to MAX_BLOCK_SIZE_GEN so the two assemblers
cannot drift on it again -- drift is what produced this.

The test is ordered before ConnectInputs on purpose: ConnectInputs credits
nFees by reference on success, so declining a transaction after calling it
would leave the coinbase claiming a fee the block does not collect.

Assembly only. CheckBlock, CheckTransaction and AcceptTransaction are
untouched: this changes what we produce, not what the network accepts.

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