RPC updates for ed/x25519 keys

[jagerman]

Edit: obsolete, see below.

• Revamps the get_all_service_nodes_public_keys RPC call to support
  returning any of the three pubkey types, and make the base32z
  conversion an opt-in option rather than forced on.

• Simplify the code by making the above RPC code just go through the
  full sninfo retrieval. Getting the list of all pubkeys doesn't seem
  to be super performance critical (it's only used for this rpc call),
  so this removes the specialized get_all_service_nodes_public_keys
  member functions from core/sn_list and just goes through the existing
  full path instead.

• adds the ed25519 and x25519 pubkeys for the current node to the rpc
  call that returns the node's own SN pubkey.

[jagerman]

Don't worry about this for release, nothing will depend on this for 5.x anyway.

[jagerman]

Going to revisit this -- I think we can make lokinet use get_n_service_nodes instead and either add a flag to base38z the pubkey values, or else do the base38z conversion in lokinet. [Edit: Jeff has said the base38z conversion in lokinet is trivially easy]

Since lokinet hits that rpc fairly often (every 5s) I think we also want another response parameter "blockchain_height" for which the SN list applies, and a request parameter "if_height_changed" that takes the last height received and, if no different than the current height, just returns a mostly empty "not_change" response.

[jagerman]

Rebased and updated this PR to do the things mentioned above.

In particular, get_n_service_nodes now accepts a "if_block_not_equal" parameter that can be given a block hash and then only returns results if the current block hash doesn't match the given one.

The main point here is to be able to give lokinet a "nothing has changed" response when it polls for new SN keys; it just needs to request the block_hash in fields on the first request, then on subsequent request can pass the currently stored block_hash in the if_block_not_equal parameter. If the block hash hasn't changed the response includes "unchanged": true and omits serializing and returning (or even calculating) all the data.

This PR also removes the base32z encoding code (Jeff is fine with just doing that in lokinet from hex values), and adds the missing ed25519/x25519 public keys to the rpc call to retrieve the SN's own pubkeys.

For example:

$ curl -X POST http://localhost:38157/json_rpc -d '
{"jsonrpc":"2.0","id":"0","method":"get_n_service_nodes",
"params":{
"if_block_not_equal":"9d13b6b0f18bfa7d965c2d0f784c7985cf45917f4e33449466b8950957f06ff9b",
"fields":{"service_node_pubkey":true}}}' -H 'Content-Type: application/json'
{
  "id": "0",
  "jsonrpc": "2.0",
  "result": {
    "block_hash": "d13b6b0f18bfa7d965c2d0f784c7985cf45917f4e33449466b8950957f06ff9b",
    "service_node_states": [{
      "service_node_pubkey": "9c09fcdb15b07ce15894b861cac270a30d190961031d6176dd51c314385d2ff5"
    },{
      "service_node_pubkey": "1c9cfead855a841a338800b77f9808f3aaae4c1bdb6a25b91ecac809506c143c"
    },{
      (etc.)
    }],
    "status": "OK",
    "unchanged": false
  }
}

but with the correct current block hash provided gives just:

$ curl -X POST http://localhost:38157/json_rpc -d '{"jsonrpc":"2.0","id":"0","method":"get_n_service_nodes", "params":{"if_block_not_equal":"d13b6b0f18bfa7d965c2d0f784c7985cf45917f4e33449466b8950957f06ff9b","fields":{"service_node_pubkey":true}}}' -H 'Content-Type: application/json'
{
  "id": "0",
  "jsonrpc": "2.0",
  "result": {
    "status": "OK",
    "unchanged": true
  }
}

[jagerman]

Closing this PR - I've merged it (and resolved some conflict/overlap) into #936