Bytes map encoding does not match protocol spec

In native_protocol_v4.spec, [bytes map] is defined as having [bytes] values, where [bytes] is an int n followed by n bytes.

In practice, CBUtil#writeBytesMap encodes each value with CBUtil#writeBytes, which writes the length as a short (so a [short bytes] instead of a [bytes]).

This is used for the custom payload introduced by CASSANDRA-8553.