PDC: persist discovery responses

When we use TCPPING.initial_hosts=A, and have nodes A, B and C, then the following can happen:

• A is started. The view is {A}
• B is started, finds A and joins the cluster. The view is {A,B}
• C is started, finds A and joins the cluster. The view is now {A,B,C}
• A is killed. The view is now {B,C}
• C is restarted
  --> C won't find A therefore cannot ask B to join it (C doesn't know about B)

SOLUTION:

• If every member persisted its discovery results on disk, C would still know about A and B, and could therefore discover B
• This could be implemented directly in TCPPING, or as a generic, separate, protocol (possibly useful for other discovery protocols such as S3_PING, TCPGOSSIP (when the GossipRouter is down) as well)
• The disk cache would have to include an expiry mechanism, so that the file doesn't grow forever, and also stale results aren't returned forever
• This mechanism would not work though if C came up the first time (no file on disk yet) and A wasn't running.