Extract and store v-key values from received messages

[fonic]

This updates the message parser to extract and store v-key values (client identifier + version info). Contents are useful e.g. to help analyze incompatibilities with certain clients, e.g. when receiving unparseable message one can check if this is related to a certain client software.

Additions/changes:

• modifies parse_message() to extract v-key value from received messages and store in message struct
• modifies new_node() to include v-key value in parameter list and to store v-key value in node struct for both instant arrival (new node is added after receiving pong message) and late arrival (new node is added after 'Nodes found', but pinged later)
• update all calls to new_node() to account for new parameter list
• modifies dump_buckets() to include v-key values

NOTE:

• This is based on my PR #37 to make use of/extend the enhanced debug output features
• If PR #31 gets merged, this has to be updated/migrated as well

Screenshot:

[jech]

Sorry, the logging infrastructure won't go in.