Mt safe

[nolleh]

associated with #21

The pull request contains below.

1. The boost.asio using pattern - completion handler / intermediate handler
   Socket operation / some members need to be serialized.
   I wrapped them with strand.
   Reference : http://stackoverflow.com/questions/12794107/why-do-i-need-strand-per-connection-when-using-boostasio
2. data accessing thread safety
   For data race condition, I've changed RedisClientImpl.state to atomic.
   If you want to serialize logic accessing state, you need to use other mechanism.

[nekipelov]

Thanks for your work!

I made benchmark (https://gist.github.com/nekipelov/d08148338716f65875bfa292181e0bf4) and got the following results:

original: ~ 25k OPS
with your code: ~ 12k OPS

This is not good. You can do mt-safe and fast?

[rpopescu]

More importantly, does this make sense?
I don't think see the reason for this use case.
In my experience, the library and several redis connections can be used in parallel just fine in a single threaded, async IO manner without the need for expensive synchronisation.