Reinstate GetMinFee DoS checks #2036

@gavinandresen 's mining refactor eliminated the check for GetMinFee on transactions included in blocks. Whether that was intentional or not, I think it's important to reinstate some of the DoS rules - particularly the one for very small outputs. Perhaps the way to do this is to add the check for GetMinFee back in, and skip anything we don't want checked in GetMinFee with GMF_BLOCK mode?

My mining refactor made GMF_BLOCK dead code (and I should have removed it), but the DoS rules for excluding transactions from the memory pool remain in place.

We can reopen discussion of what criteria should be used for selecting transactions, but I don't think special-casing "very small outputs" any more than we do now (transactions with outputs less than MIN_RELAY_TX_FEE must pay a fee or they won't be added to the memory pool) is the right criteria.

With unspent TxOuts being the thing we want to minimize, making transaction priority higher for transactions with a better #inputs/#outputs ratio seems like a better way to go, although last time around we couldn't seem to reach consensus on exactly what the formula should be.

MIN_RELAY_TX_FEE was originally intended to only ease reducing MIN_TX_FEE in the future by relaying them but not including them in blocks. The drop to 0.0005 BTC fees was the result of 1 BTC = $25, so if anything the logical step would be to increase the minimums to 0.001 BTC, not decrease it to 0.0001 BTC.

Rethinking how we apply this or other anti-DoS rules is fine, but right now we seem to have reduced the limits so that spam can get into the free areas more easily than it had been intended(? - or at least discussed). Considering blk0002 is nearly full in a very short amount of time, we should probably do something.

Closing, I believe the dust rule covers this.