Include Recent Blockhash in each Transaction

[aakilfernandes]

Specification

In each transaction (prior to signing), include the following

1. The blockhash of a recent block
2. A single byte blocklimit

In order for a transaction to be valid, it must be included in the blockchain within blocklimit blocks of a block with hash blockhash. Transactions with a blocklimit and blockhash of 0 are always valid, regardless of the chain history.

Reasoning

This would offer a number of improvements

1. It would prevent transactions on one chain from being processed on another chain. This would mean the same private keys could be used on both chains.
2. It would provide definitive proof that a transaction would not be included in the chain (you would know within blocklimit blocks a transaction would not be broadcast, therefore you could safely ignore it).

[aakilfernandes]

Github done goofed and for some reason this got submitted twice: #134