Protocol changes for SPV clients: mempool, getdata commands

The proposed change involves adding a 'mempool' P2P command and extending the 'getdata' behavior. Wladimir fully supports the changes but suggests creating a small Bitcoin Improvement Proposal (BIP) for consistency and documentation purposes.The proposal aims to add a new command called 'mempool' to the satoshi client v0.7. This addition would benefit simplified payment verification (SPV) clients by providing them access to the memory pool. It would also allow diagnostics and miner download capabilities. Additionally, the 'getdata' behavior would be extended to include any memory pool transaction. This change would relax a previous restriction on the 'getdata' behavior.Although the proposal does not necessarily require a BIP, Jeff Garzik has offered to write one if deemed necessary. This step would ensure consistency and provide documentation for other client developers. Overall, the proposed changes aim to enhance the functionality and accessibility of the satoshi client v0.7.