Pull Request #597 Don't hang for dns error responding to fragmented requests
If hostnames are used instead of ip addresses for all hosts within a pool,
and dns lookup fails, then get/multiget will hang indefinitely **instead of**
responding with an error such as `SERVER_ERROR Host is down`.
(I expect a client would detect this and close the connection,
but this is not ideal, the client timeout could be a second)
- Both redis and memcache protocols are affected
- If a server resolves but is down, then `get` does respond with `SERVER_ERROR Host is down`.
I suspect that's because memcached get is implemented to coalesce responses
from multiple backend servers, even when there's only one key,
and this is likely a bug specific to handling coalescing when
there's no attempt to send the request to a backend server
Because connection attempts are async but dns lookup is asynchronous,
there's a non-null server connection for a host that's unavailable
but a null server connection for a host that has a dns lookup error,
and these end up using different code paths.
(e.g. the former will call server_close() which does adjust nfrag_sent)
Fixes #596
