MINOR: peers: Make peers protocol support new "server_name" data type.

Make usage of the APIs implemented for dictionaries (dict.c) and their LRU caches (struct dcache)
so that to send/receive server names used for the server by name stickiness. These
names are sent over the network as follows:

 - in every case we send the encode length of the data (STD_T_DICT), then
 - if the server names is not present in the cache used upon transmission (struct dcache_tx)
   we cache it and we the ID of this TX cache entry followed the encode length of the
   server name, and finally the sever name itseft (non NULL terminated string).
 - if the server name is present, we repead these operations but we only send the TX cache
   entry ID.

Upon receipt, the couple of (cache IDs, server name) are stored the LRU cache used
only upon receipt (struct dcache_rx). As the peers protocol is symetrical, the fact
that the server name is present in the received data (resp. or not) denotes if
the entry is absent (resp. or not).
4 files changed