Add infrastrcuture for creating a block template.

This commit adds a new function named NewBlockTemplate along with
supporting infrastructure which is part of the core functionality needed
to support mining.

In particular the function creates a new block template which contains a
fully populated block with a zero nonce that is ready to be solved as well
as additional information regarding the fees and number of signature
operations for each transaction included in the block.  The specific
transaction selection logic mirrors the reference implementation.

Various cleanup and suggestions provided by @owainga.