remove fixed rx buffer allow definition per protocol

remove fixed rx buffer allow definition per protocol

A new protocol member is defined that controls the size of rx
buffer allocation per connection.  For compatibility 0 size
allocates 4096, but you should adapt your protocol definition
array in the user code to declare an appropriate value.

See the changelog for more detail.

The advantage is the rx frame buffer size is now tailored to
what is expected from the protocol, rather than being fixed
to a default of 4096.  If your protocol only sends frames of
a dozen bytes this allows you to only allocate an rx frame
buffer of the same size.

For example the per-connection allocation (excluding headers)
for the test server fell from ~4500 to < 750 bytes with this.

Signed-off-by: Andy Green <andy.green@linaro.org>