refactoring mysql store code

weed/filer/mysql_store/README.md

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+#MySQL filer mapping store
+
+## Schema format
+
+
+Basically, uriPath and fid are the key elements stored in MySQL. In view of the optimization and user's usage, 
+adding primary key with integer type and involving createTime, updateTime, status fields should be somewhat meaningful.
+Of course, you could customize the schema per your concretely circumstance freely.
+
+<pre><code>
+CREATE TABLE IF NOT EXISTS `filer_mapping` (
+  `id` bigint(20) NOT NULL AUTO_INCREMENT,
+  `uriPath` char(256) NOT NULL DEFAULT "" COMMENT 'http uriPath',
+  `fid` char(36) NOT NULL DEFAULT "" COMMENT 'seaweedfs fid',
+  `createTime` int(10) NOT NULL DEFAULT 0 COMMENT 'createdTime in unix timestamp',
+  `updateTime` int(10) NOT NULL DEFAULT 0 COMMENT 'updatedTime in unix timestamp',
+  `remark` varchar(20) NOT NULL DEFAULT "" COMMENT 'reserverd field',
+  `status` tinyint(2) DEFAULT '1' COMMENT 'resource status',
+  PRIMARY KEY (`id`),
+  UNIQUE KEY `index_uriPath` (`uriPath`)
+) DEFAULT CHARSET=utf8;
+</code></pre>
+
+
+The MySQL 's config params is not added into the weed command option as other stores(redis,cassandra). Instead,
+We created a config file(json format) for them. TOML,YAML or XML also should be OK. But TOML and YAML need import thirdparty package
+while XML is a little bit complex.
+
+The sample config file's  content is below:
+
+<pre><code>
+{
+    "mysql": [
+        {
+            "User": "root",
+            "Password": "root",
+            "HostName": "127.0.0.1",
+            "Port": 3306,
+            "DataBase": "seaweedfs"
+        },
+        {
+            "User": "root",
+            "Password": "root",
+            "HostName": "127.0.0.2",
+            "Port": 3306,
+            "DataBase": "seaweedfs"
+        }
+    ],
+    "IsSharding":true,
+    "ShardingNum":1024
+}
+</code></pre>
+
+
+The "mysql" field in above conf file is an array which include all mysql instances you prepared to store sharding data.
+1. If one mysql instance is enough, just keep one instance in "mysql" field.
+2. If table sharding at a specific mysql instance is needed , mark "IsSharding" field with true and specify total table
+sharding numbers using "ShardingNum" field.
+3. If the mysql service could be auto scaled transparently in your environment, just config one mysql instance(usually it's a frondend proxy or VIP),
+and mark "IsSharding" with false value
+4. If your prepare more than one mysql instances and have no plan to use table sharding for any instance(mark isSharding with false), instance sharding
+will still be done implicitly
+
+
+
+