Provide a faster alternative to UUID

[Stebalien]

UUIDs are slow. They require allocation and a systemcall. We should be using an instance unique UUID plus a global counter:

type EventId struct {
    Inst: UUID // we can rotate this if we overflow uint64.
    Ctr: uint64
}

[Kubuxu]

@Stebalien uuid lib we are using uses crypt/rand for uuid generation: https://github.com/satori/go.uuid/blob/master/uuid.go#L113-L117

[Stebalien]

Yep. On Linux, at least, that always makes a systemcall (although I guess it doesn't allocate, it uses a fixed size byte array). Systemcalls are expensive. For now, we aren't doing anything where this would make a difference. However, if we switch to an RPC-based API with suffucient buffering, we could theoretically get down to <1 syscall per RPC. At that point, generating a UUID becomes expensive.

[Kubuxu]

That is true, Read from crypt/rand uses getrandom(2).