Stellar Asset Contract Issuer semantics

[sisuresh]

The Stellar Asset Contract special cases the issuer because issuers can't have a trustline to their own asset, which leads to two related details that should be revisited -

1. If the issuer is the sender in a transfer, the amount will be minted. If the issuer is the receiver in a transfer, the amount will be burned. This follows classic Stellar semantics.
2. In the two above cases where the token is minted or burned using transfer, we emit the transfer event (instead of mint or burn).

These two issues lead to additional difficulties when it comes to tracking token balances. We have two actions we can take here -

1. Stop special casing the issuer in the SAC and fail when the issuer is the sender or receiver.
2. Continue allowing the issuer case, but emit mint or burn instead of transfer when the issuer is involved.