Rearrange the channel buffer sizes into three neat use-editable values in options.h. Increasing RECV_MAX_WINDOW gives big network performance increases - even with the present buffers (which haven't changed) it performs a lot better. Next step is to make the window size a cmdline option.

256-bits (k = 36113) = 115792089237316195423570985008687907853269984665640564039457584007913129603823
512-bits (k = 38117) = 13407807929942597099574024998205846127479365820592393377723561443721764030073546976801874298166903427690031858186486050853753882811946569946433649006045979