add impl for ICapTransaction

src/DotNetCore.CAP/ICapTransaction.Base.cs

+using System.Collections.Generic;
+using DotNetCore.CAP.Models;
+
+namespace DotNetCore.CAP
+{
+    public abstract class CapTransactionBase : ICapTransaction
+    {
+        private readonly IDispatcher _dispatcher;
+
+        private readonly IList<CapPublishedMessage> _bufferList;
+
+        protected CapTransactionBase(IDispatcher dispatcher)
+        {
+            _dispatcher = dispatcher;
+            _bufferList = new List<CapPublishedMessage>(1);
+        }
+
+        public bool AutoCommit { get; set; }
+
+        public object DbTransaction { get; set; }
+
+        protected internal void AddToSent(CapPublishedMessage msg)
+        {
+            _bufferList.Add(msg);
+        }
+         
+        protected void Flush()
+        {
+            foreach (var message in _bufferList)
+            {
+                _dispatcher.EnqueueToPublish(message);
+            }
+        }
+
+        public abstract void Commit();
+
+        public abstract void Rollback();
+
+        public abstract void Dispose();
+    }
+}