Add SingleGossip commitment level to use for subscriptions

[CriesofCarrots]

Problem

#10137 investigates triggering bank-related notifications from gossip, but the notification thread will get bogged down if it has to check the entire notifications list twice per slot. It would be better to separate subscriptions that should be notified via gossip and those that should be notified vie replay stage, so they can be checked and notified separately.

Summary of Changes

• Add CommitmentLevel::SingleGossip
• Add gossip account, program, and signature subscription lists, and channel SingleGossip subscriptions there
• Add method to notify_gossip_subscribers, to be used in Trigger notifications on supermajority votes confirmation #10137

Open Questions:

• Do we need to support the SingleGossip commitment level in rpc.rs? If so, how will we get the most recent 2/3 slot from cluster_info_vote_listener?
• Is there a better variant name than SingleGossip?

[codecov]

Codecov Report

Merging #10147 into master will increase coverage by 0.0%.
The diff coverage is 94.8%.

@@           Coverage Diff           @@
##           master   #10147   +/-   ##
=======================================
  Coverage    81.4%    81.5%           
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  Files         288      283    -5     
  Lines       66211    66176   -35     
=======================================
+ Hits        53938    53949   +11     
+ Misses      12273    12227   -46     

[CriesofCarrots]

@mvines , can you take a quick peek at this? This lays the groundwork for #10137 , which implements notifications on seeing 2/3+ stake votes in gossip.
In particular, I would like your take on my open questions above.
Thank you!

[jstarry]

I think this is great that we are moving towards the direction of only checking subscriptions that need to be checked!

I think a more generalized solution could be implemented (now or later) where we track the last checked slot for a particular commitment level. Right now, it looks like Max, Root, and Single could all be double checked in the current implementation. A nice side effect of tracking the last checked slot for each commitment level would be that we could use it to initialize last_notified_slot for account subscriptions which seems kind of tricky for single gossip

[jstarry]

Might be time to add a subscription id lookup map, thoughts?

[CriesofCarrots]

As discussed on slack, agreed! Will file an issue for follow-up refactoring work.