add rabbitmq connection pool.

src/DotNetCore.CAP.RabbitMQ/ConnectionPool.cs

+using System;
+using System.Collections.Concurrent;
+using System.Diagnostics;
+using System.Threading;
+using RabbitMQ.Client;
+
+namespace DotNetCore.CAP.RabbitMQ
+{
+    public class ConnectionPool : IConnectionPool, IDisposable
+    {
+        private const int DefaultPoolSize = 32;
+
+        private readonly ConcurrentQueue<IConnection> _pool = new ConcurrentQueue<IConnection>();
+
+        private readonly Func<IConnection> _activator;
+
+        private int _maxSize;
+        private int _count;
+
+        public ConnectionPool(RabbitMQOptions options)
+        {
+            _maxSize = DefaultPoolSize;
+
+            _activator = CreateActivator(options);
+        }
+
+        private static Func<IConnection> CreateActivator(RabbitMQOptions options)
+        {
+            var factory = new ConnectionFactory()
+            {
+                HostName = options.HostName,
+                UserName = options.UserName,
+                Port = options.Port,
+                Password = options.Password,
+                VirtualHost = options.VirtualHost,
+                RequestedConnectionTimeout = options.RequestedConnectionTimeout,
+                SocketReadTimeout = options.SocketReadTimeout,
+                SocketWriteTimeout = options.SocketWriteTimeout
+            };
+
+            return () => factory.CreateConnection();
+        }
+
+        public virtual IConnection Rent()
+        {
+            if (_pool.TryDequeue(out IConnection connection))
+            {
+                Interlocked.Decrement(ref _count);
+
+                Debug.Assert(_count >= 0);
+
+                return connection;
+            }
+
+            connection = _activator();
+
+            return connection;
+        }
+
+        public virtual bool Return(IConnection connection)
+        {
+            if (Interlocked.Increment(ref _count) <= _maxSize)
+            {
+                _pool.Enqueue(connection);
+
+                return true;
+            }
+
+            Interlocked.Decrement(ref _count);
+
+            Debug.Assert(_maxSize == 0 || _pool.Count <= _maxSize);
+
+            return false;
+        }
+
+        IConnection IConnectionPool.Rent() => Rent();
+
+        bool IConnectionPool.Return(IConnection connection) => Return(connection);
+
+        public void Dispose()
+        {
+            _maxSize = 0;
+
+            IConnection context;
+            while (_pool.TryDequeue(out context))
+            {
+                context.Dispose();
+            }
+        }
+    }
+}

src/DotNetCore.CAP.RabbitMQ/IConnectionPool.cs

+using System;
+using System.Collections.Generic;
+using System.Text;
+using RabbitMQ.Client;
+
+namespace DotNetCore.CAP.RabbitMQ
+{
+    public interface IConnectionPool
+    {
+        IConnection Rent();
+
+        bool Return(IConnection context);
+    }
+}