add connection pool size config to KafkaOption.

src/DotNetCore.CAP.Kafka/CAP.KafkaOptions.cs

 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 
@@ -16,16 +17,21 @@ namespace DotNetCore.CAP
         /// Topic configuration parameters are specified via the "default.topic.config" sub-dictionary config parameter.
         /// </para>
         /// </summary>
-        public readonly IDictionary<string, object> MainConfig;
+        public readonly ConcurrentDictionary<string, object> MainConfig;
 
         private IEnumerable<KeyValuePair<string, object>> _kafkaConfig;
 
 
         public KafkaOptions()
         {
-            MainConfig = new Dictionary<string, object>();
+            MainConfig = new ConcurrentDictionary<string, object>();
         }
 
+        /// <summary>
+        /// Producer connection pool size, default is 10
+        /// </summary>
+        public int ConnectionPoolSize { get; set; } = 10;
+
         /// <summary>
         /// The `bootstrap.servers` item config of <see cref="MainConfig" />.
         /// <para>

src/DotNetCore.CAP.Kafka/ConnectionPool.cs

@@ -8,10 +8,7 @@ namespace DotNetCore.CAP.Kafka
 {
     public class ConnectionPool : IConnectionPool, IDisposable
     {
-        private const int DefaultPoolSize = 15;
-
         private readonly Func<Producer> _activator;
-
         private readonly ConcurrentQueue<Producer> _pool = new ConcurrentQueue<Producer>();
         private int _count;
 
@@ -19,8 +16,7 @@ namespace DotNetCore.CAP.Kafka
 
         public ConnectionPool(KafkaOptions options)
         {
-            _maxSize = DefaultPoolSize;
-
+            _maxSize = options.ConnectionPoolSize;
             _activator = CreateActivator(options);
         }