fix concurrency issue

[irgendwr]

fixes #21

[stevenh]

I'm in too minds if we want to do this, it adds the lock overhead to a client which can only perform one request at a time. This falls under the remit of nothing being concurrent safe unless documented to be so.

[irgendwr]

I can try to think of a better solution to solve this but it should definitely be concurrent safe imho (the fact that two people reported the issue suggests that it's expected despite not being "documented to be so").
Is the tiny overhead really that significant in this use case?

[RonMelkhior]

My issue doesn't come from trying to use the library from multiple goroutines at once.

My issue happens once unhandled timeout/connection closed errors occur. (for example: read: connection reset by peer)

Because the next time I execute a command on that closed connection, the conn.Write() call freezes indefinitely, causing the issue.

[stevenh]

enabling keepalive on the connection should fix this

[irgendwr]

conn.Write() should also never freeze "indefinitely", only as long as the Timeout that is specified/DefaultTimout, since there's a Deadline set before calling it.