Remove double-spends from mempool.

This patch adds a check to clear the mempool of nodes after each new block, removing double-spends of the outputs that have just been spent in the block.  In addition, the effect of "name_clean" is now performed automatically at each new block for the user's wallet.  "name_clean" as explicit command is removed, since it has no longer any additional functionality.

It is possible (and plausible according to my current understanding) that this is the final fix for the "stuck players" problem - note, however, that both most of the network (for relaying transactions) and especially the miners have to upgrade for the fix to become effective.

Please test this a bit to make sure no bugs have been introduced.  I've been able to reproduce a situation with the reported symptoms (no longer able to get transactions confirmed after deleting a move transaction) on a private testnet, and can confirm that this is fixed by this patch.  I also believe that I understand the cause for the stuck players, and according to my understanding, this patch should be the right one.  However, since I've not experienced stuck players myself so far, I can not be 100% sure that the issue fixed here is the main one - but in any case, the patch improves the situation.