Prepare for release (opam file et al)

[vbmithr]

@mfp What about those changes?

[mfp]

Could you rewrite the last few entries of the history and extract #699ba5f929ceb4c so we can consider it independently? I'd like to merge everything else, but I see 2 issues with that commit (which moreover is out of scope for this PR):

• the parsing of peers in dict.ml cannot possibly work for IP:port addresses, since : is used as both the node and IP:port separator
• the change to Oraft_rsm seems to preclude cluster membership changes altogether? We can support both settings (static cluster and dynamic membership) easily, and then use the static configuration in dict for simplicity:

    begin match cluster with
      | `Static (peers, passive) ->
          Lwt.return (Simple_config ((id, addr) :: peers, passive), None)
      | `Join peer_addr ->
          ...

[vbmithr]

Done!