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

We only retry with 3 seconds intervals. 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.