hare3: support multiple smeshers

[dshulyak]

the end goal is to support multiple identities in hare3 protocol

storing multiple smeshers and registration

type Hare needs to store collection of smeshers, this collection is extended with a Register(Smesher) call.

type Protocol interface {
    Register(*EdSigner)
}

Register can't fail, so if atx doesn't exist yet it simply registers smesher in the internal collection.

in order to prevent new participants joining mid session when hare session starts, session needs to copy pointers to registered smeshers at that time and store them in different collection.

eligibility and emitting messages

computing ecvrf is ~200us. ed25519 sig is ~100us.

lets assume that we target 10_000 identities. that is about 3s of on cpu time for every message. we need to parallelize that to support this amount. the efficient approach would be to create N worker goroutines managed by hare. each worker will accept batch of vrf's to compute / messages to sign and output result. this is also good change that can be submitted after initial implementation to simplify review.

we also should disable vrf / sig verification which is run when nodes Publish message. to implement check that message is emitted by our own host.ID() and do the necessary skip.

to vote on the same set of proposals, proposals method should be modified to not vote on the proposal if atx that belongs to any smesher is lower than the proposals base height.

otherrefactoring

reference to vrf signer needs to be moved out of the legacy oracle https://github.com/spacemeshos/go-spacemesh/blob/develop/hare/eligibility/oracle.go#L75