Check wrong metadata_size values during peer extended handshake

Sometimes, some peers give us a value too large for metadata_size during
the handshake process. Those _extremely high_ values were the root (or
part of) of those random hangs while downloading magnets that some of us
were experiencing lately.

This patch adds a check to close the connection with the peers that send
us those metadata_size values during the handshake process. Now, the
maximum value allowed for the metadata is 4MiB.