If -spkreuse=0, ensure transactions in mempool always have unique scriptPubKeys

[luke-jr]

Exceptions:

• Multiple inputs in the same transaction are allowed to spend against the same scriptPubKey
• The same scriptPubKey may be used in the mempool as both first an output, and then spent in a later transaction's input

Changes since original 2013 patch (pre-squashed):

• Refactor: Move CScript::ScriptPubkeyReuseHash to ScriptHashkey(CScript) in txmempool
• Update mempool duplicate-scriptPubKey limiting with C++11 and misc formatting improvements
• Bugfix: Use bitwise operators for mempool SPK states
• Use CValidationState for SPK reuse rejections
• Move mapTxSPK to CTxMemPoolEntry.mapSPK
• Make SPK reuse filtering optional (use -spkreuse)

Known issues:

• This breaks RBF in most usage scenarios.
• Someone could watch for transactions and spam dust to block them on nodes using this.

[luke-jr]

@sdaftuar pointed out on IRC that someone could watch the p2p network and screw up people by sending dust to SPKs they see. I also noticed this breaks common RBF usage since the SPK conflict rejects the transaction before it can be replaced.

To address both of these, I refactored the logic so that it treats non-inherent (that is, within the same tx) SPK conflicts in the same manner as TxIn conflicts. Although in the SPK case, the RBF optin flag is ignored, and a conflict in the parents of the transaction don't cause a DoS ban.

[morcos]

As mentioned on IRC, I'm opposed to this feature. Would not object to more optional safeguards against reuse in the wallet code though.

[luke-jr]

And as mentioned on IRC, I don't like the idea of changing Core's direction to where it is no longer a reference implementation, but a specific political agenda to the exclusion of others. If you don't like the feature, just don't use it.

[sdaftuar]

@luke-jr Please explain in the pull request, and ideally in code comments, what this patch is intended to do and why. Concept NACK policy change PRs that are made without providing motivation.

[luke-jr]

Context: https://bitcointalk.org/index.php?topic=334316.0;viewResults