netsync: Rework external request from peer.

[davecgh]

This requires #3499.

This reworks the logic that allows code external to the sync manager to request blocks, votes, and treasury spend transactions to both simplify and optimize it. The method now makes use of the same logic the sync manager itself uses when requesting data and no longer potentially returns an error.

Of particular note is that it now uses the same much more efficient code path that the sync manager itself uses to determine if a transaction is needed as opposed to the rather expensive legacy utxo-based query approach.

Finally, this also modifies the logic to potentially make more than one request if needed since it is technically more accurate and avoids any possibility of error even though the current request sizes are nowhere near the limits.

[jholdstock]

Nice improvement! Is numRequested actually necessary? Cant you just use len(gdMsg.InvList)? Is there overhead associated with calling len?

[davecgh]

I haven't tested it in a long time, so there might not be anymore, but there used to be extra overhead. I really just followed the same pattern used in the other paths that do the same thing.