Fix Do("") handling

[tysonmote]

redigo/redis supports a special blank command in Do which flushes the connection's output buffer and receives all pending replies (if any). redigo/redis uses this when a connection is returned to its pool to ensure that the connection is ready for re-use no matter what state it is returned in.

This commit fixes the redisc connection's Do wrapper when called with the blank command:

• If the connection is not currently bound to a node, it is not bound to a random node anymore. It immediately returns, instead.

• If the connection is bound to a node, do not re-bind to another node. Pass along the special blank command to the underlying redigo/redis connection.

[mna]

Thanks, LGTM!

[mna]

Oops, no one thing though, the rc field cannot be accessed without locking the connection's mutex, I'll make the change.