Add LokiAPI.ping(_:)

SignalServiceKit/src/Loki/LokiAPI.swift

@@ -57,6 +57,10 @@ import PromiseKit
         return getRandomSnode().then { invoke(.sendMessage, on: $0, with: lokiMessage.toJSON()) } // TODO: Use getSwarm()
     }
     
+    public static func ping(_ hexEncodedPublicKey: String) -> Promise<RawResponse> {
+        return getRandomSnode().then { invoke(.sendMessage, on: $0, with: [ "destination" : hexEncodedPublicKey ]) } // TODO: Use getSwarm() and figure out correct parameters
+    }
+    
     public static func getSwarm(for hexEncodedPublicKey: String) -> Promise<Set<Target>> {
         return getRandomSnode().then { invoke(.getSwarm, on: $0, with: [ "pubKey" : hexEncodedPublicKey ]) }.map { rawResponse in return [] } // TODO: Parse targets from raw response
     }

SignalServiceKit/src/Loki/LokiMessage.swift

@@ -8,6 +8,8 @@ public struct LokiMessage {
     /// The time to live for the message.
     let ttl: UInt64
     /// When the proof of work was calculated, if applicable.
+    ///
+    /// - Note: Expressed as seconds since 00:00:00 UTC on 1 January 1970.
     let timestamp: UInt64?
     /// The base 64 encoded proof of work, if applicable.
     let nonce: String?