add producers to impacted and notify in asset_update_feed_producers_operation

[oxarbitrage]

we added fee_payer() to all operations in bitshares at: #955

we want to also add producers to asset_update_feed_producers_operation as proposed in:
#955 (comment)

why hardfork is needed here?
Abit More: adding fee_payer doesn't change account history numbering but adding producers will
so we need to do it with a hard fork guard.

[ryanRfox]

I placed this in the current Protocol Upgrade release, but it's not clear if it should be removed back to the Project Backlog at this time.

[abitmore]

Won't do this in this release.