Initial MTX Messaging layer proposal

[chrisdigity]

This commit proposes suggested validation logic for a messaging layer
that would exist as an extension to the existing MTX logic available
from v2.4.1 of the codebase. The messaging layer would allow a sender
of a transaction to append a message (known in the code as a memo) to
the last destination tag of an MTX, using any remaining space available
in the dst[] field.

Messaging layer specification:
To maintain transaction type conformity, a transaction identified as a
MTX Memo transaction SHALL have a memo exist in the transaction.
To distinguish a memo from the dst[] tags, a memo SHALL always begin
and end with a zero (0) byte (null termination char).
To avoid binary abuse of the messaging layer, a memo SHALL contain only
printable characters, as per the default C locale, and any remaining
dst[] array space AFTER the the end of a memo SHALL be zero filled.
To offset the validation costs of longer messages, the fee incurred for
a memo of a certain length shall be equal to the fees incurred by tags
that would otherwise use such a length.

The fee formula:
(messagefees) = (fee) * (1 + ((messagelength + 1) / sizeof(MDST)))

'1 + ...' ensures that a fee is always paid by messages that do not
utilize the entire space (MDST) of a dst[] field.
'... (messagelength + 1) ...' ensures the initial zero (0) byte,
which utilizes dst[] tag space, is accounted for in fee calculation.

Notes...
Currently the fee calculation does not include the required ending zero
byte terminating character. This allows a message to include a network
fee for the first 19 characters and a subsequent network fee for every
20 characters thereafter. Accounting for the ending zero byte will
introduce further (and objectively) unecessary validation processes.

[chrisdigity]

This Pull Request has been opened as a Draft and will not be made available for merge until appropriate discussion has taken place.