CLEANUP: peers: Replace hard-coded values by macros.

CLEANUP: peers: Replace hard-coded values by macros.

All the peer stick-table messages are made of a 2-byte header (PEER_MSG_HEADER_LEN)
followed by the encoded length of the remaining data wich is harcoded as 5 (in bytes)
for the maximum (PEER_MSG_ENCODED_LENGTH_MAXLEN). With such a length we can encode
a maximum length which equals to (1 << 32) - 1, which is from far enough.

This patches replaces both these values by macros where applicable.