Change signed delegate action format to avoid confusing it with other things that might be signed by an account key

[jakmeier]

Signing a meta transaction using an account key can be problematic, as the signed message could be used in a different context where account keys are also used for signing. For now, only transactions are signed with the account key but that is likely to change in the future.

[jakmeier]

In the protocol discussion today, @abacabadabacaba presented his ideas how to solve this and we reached an agreement to use the first ~4 bytes of a message to define the type of the message. Conflicts with today's representation of a transactions can be avoided because a transaction start with the length of the account ID, which is always between 2 and 64.

A NEP will have to be created to define the exact scheme and also allocate a type number to meta transactions.

Before we can stabilize meta transactions, we will have to update the implementation to be in line with this yet-to-be-written NEP.

[jakmeier]

A NEP draft for this has started now: near/NEPs#461

To work on the issue at hand, we will most likely implement whatever that NEP ends up specifying once it is finalized and approved.