The send buffers are set to 8 * MSS = ~11.5K.  MEM_SIZE is now set to
be larger than that, 16K, in order to help avoid having tcp_write fail
with ENOMEM.  The attempt to use a smaller size is still included, which
will allow a tcp_write of up to 16 * 16K = 256K, i.e. all of memory.

Fixes #725