After lwip_init() is called, you must call tcp_tmr() every
TCP_TMR_INTERVAL milliseconds (default is 250 milliseconds).
My quesiton: Is there any unpleasant side effect if I call it at a rate
other than 250ms? Further, how about randomized intervals, say 80ms,
83ms, 98ms.... (I tried and haven't notice any yet but would like to hear
from the gurus to be sure)