Respecting maximum packet size.

Respecting maximum packet size.

The header size macros for long and short packets were fixed to provide
correct values in bytes.

Currently the sending code limits frames so they don't exceed max_packet_size.
But it does not account the case when a single frame can exceed the limit.

As a result of this patch, big payload (CRYPTO and STREAM) will be split
into a number of smaller frames that fit into advertised max_packet_size
(which specifies final packet size, after encryption).