FIx multi thread calls asyncWrite concurrently.

[g-xianhui]

Multiple thread may excute asyncWrite at the same time.
1. Thread A request quite a lot RedisAsyncClient::command, all the
command post to RedisClientImpl::strand.
2. Thread B execute commands in strand one by one. The first asyncWrite
swap dataWrited and dataQueued, so other doAsyncCommand won't enter
asyncWrite.
3. the first asyncWrite call ends with a boost::asio::async_write, and
left a later asyncWrite as handler.
4. boost::asio::async_write complete, thread C excute the handler
asyncWrite, clear dataWrited.
5. Now thread B execute doAsyncCommand, and find dataWrited was empty,
so it enters asyncWrite concurrently!