Simple Subnet Management

[AgeManning]

Overview

This is an implementation of ethereum/consensus-specs#3735

Essentially, rather than having Lighthouse rotate every 24 hours around long-lived subnets, it will stay fixed on subnets for the duration of its run (and it's lifetime until a user manually deletes the network.key in the lighthouse directory).

The rotations have a dubious practical importance and adds quite a bit of complexity in our code base. By removing the rotations I was able to simplify the code. This also helps with debugging and keeping tracking of subnets and ENRs (our ENRs will also be stable, as we no longer need to rotate the subnet ids there).

While I was in there, I took the liberty to combine the SyncCommitteeSubnets with the AttestationSubnets into a single SubnetService which I think is a bit cleaner and easier to reason about.

Because this PR changes a bit of code and if there are bugs will immediately impact the performance of validators, we should take caution in merging this before significant testing.

[AgeManning]

Thanks for the review @jimmygchen - I think i've addressed all of these

[jimmygchen]

I've reviewed the PR and it looks much cleaner! I've added a few comments / questions.
I think this will simplify implementing PeerDAS validator custody (subscriptions) too.

[jimmygchen]

Nice, looks good to me now! 👍