useUnicode 默认为 true
characterEncoding 仅在 useUnicode=true 时起作用，默认 autodetect，常设为 UTF-8
autoReconnect 默认 false，官方：
The use of this feature is not recommended,
because it has side effects related to session state and data consistency when applications don't handle SQLExceptions properly,
and is only designed to be used when you are unable to configure your application to handle SQLExceptions resulting from dead and stale connections properly.
autoReconnectForPools 默认 false
useServerPrepStmts 是否开启服务端 prepared statements，默认 false。配合 cachePrepStmts=true 等配置，缓存 sql 预编译脚本，提升执行效率；相应会多占用内存
cachePrepStmts=true&prepStmtCacheSize=250&prepStmtCacheSqlLimit=2048 开启缓存预编译的 sql，根据 LRU 策略最大缓存 250 条，只缓存长度在 2048 以下的 sql
useSSL 某些版本 MySQL 需要显示声明 true/false
