Not possible to determine if a signer was updated or added from operation result and meta

[nullstyle]

At present it is very tough in the context of the horizon importer to determine whether or not a signer was added or updated within a SetOptionsOp operation.

To determine whether the modified signer was updated or created for the first time, the only feasible methods are:

1. find the previous state of the account by loading it from the history system, somehow.
2. search back through horizon's imported history api for the previous state of the signer

Both of these seem to be very costly and/or cludgy. Ideally, we could just add a flag on the SetOptionsResult that tells me whether a signer was added/updated/removed as a result of the operation being applied.

[nullstyle]

#712 contains one possible way to improve this situation.