Feature/websocket sequencing (#267)

Added EnforceSequenceNumbers property on SocketApiClient to configure whether websocket message contain sequence numbers and if these should be checked to be sequential
Added fallback to existing websocket connection if no dedicated request connection was found
Added IntBoolConverter base class for arbitrary int value to bool mapping
Added SequenceNumber property to DataEvent object
Added _skipSequenceCheckFirstUpdateAfterSnapshotSet property for SymbolOrderBook implementations
Updated SymbolOrderBook sequenceNumber validation
Updated SymbolOrderBook log verbosities
Renamed SetInitialOrderBook to SetSnapshot in SymbolOrderBook
Renamed updateId references to sequenceNumber in SymbolOrderBook

CryptoExchange.Net/Clients/SocketApiClient.cs

@@ -142,6 +142,10 @@ namespace CryptoExchange.Net.Clients
         /// </summary>
         public int? MaxIndividualSubscriptionsPerConnection { get; set; }
 
+        /// <summary>
+        /// Whether or not to enforce that sequence number updates are always (lastSequenceNumber + 1)
+        /// </summary>
+        public bool EnforceSequenceNumbers { get; set; }
         #endregion
 
         /// <summary>
@@ -706,6 +710,10 @@ namespace CryptoExchange.Net.Clients
                 if (connection != null && !connection.DedicatedRequestConnection.Authenticated)
                     // Mark dedicated request connection as authenticated if the request is authenticated
                     connection.DedicatedRequestConnection.Authenticated = authenticated;
+
+                if (connection == null)
+                    // Fall back to an existing connection if there is no dedicated request connection available
+                    connection = socketQuery.OrderBy(s => s.UserSubscriptionCount).FirstOrDefault();
             }
 
             bool maxConnectionsReached = _socketConnections.Count >= (ApiOptions.MaxSocketConnections ?? ClientOptions.MaxSocketConnections);