Optimize for outgoing message checksum.

[evoskuil]

Incoming message checksum can simply be disabled, which is the default configuration, but outgoing cannot, so:

Use the existing tx hash to obtain for the message checksum. This is the only message object that would have a precomputed bitcoin hash, but is is also the most common larger object.

Cache checksums on message objects and parameterize in send so that they are not recreated for each send of the same instance.

If checksum validation is enabled, cache the block value onto the message object for storage without the need for re-computation.