Refresh topology in case of generic errors in Do().

This fixes the cases where Do() didn't handle failed over nodes. Because
in case of failing over a node, gomemcached return EOF, in case of
killing the node's couchbase process, we get connection errors. Currently,
in these cases client will try to refresh the topology every 3 seconds.
This way, when we manually fail the node over or automatic failover kicks
in, topology is refreshed and everything works normally. Previously, it
was returning errors forever until a rebalance was done, causing a
NOT_MY_VBUCKET error.