MINOR: peers: A bit of optimization when encoding cached server names.

MINOR: peers: A bit of optimization when encoding cached server names.

When a server name is cached we only send its cache entry ID which has
an encoded length of 1 (because smaller than PEER_ENC_2BYTES_MIN).
So, in this case we only have to encode 1, the already known encoded length
of this ID before encoding it.

Furthermore we do not have to call strlen() to compute the lengths of server
name strings thanks to this commit: "MINOR: dict: Store the length of the
dictionary entries".