update token fill frequency

[chen042531]

update tokens every 8000000ns
use 8000000ns because the minimum token rate is 1 Kbit/s (125 bytes/s)
add at least one token in every update

[tim-ywliu]

#define REFILL_TOKEN_INTERVAL 1000000 // ns (=1ms), 1 token = 1 byte
#define SECOND_TO_NANOSECOND 1000000000

if (p->refillTokenTime >= REFILL_TOKEN_INTERVAL) {
        // add at least one token
        p->refillTokenTime = p->refillTokenTime - REFILL_TOKEN_INTERVAL;
        tokensToAdd = p->tokenRate * (REFILL_TOKEN_INTERVAL  / SECOND_TO_NANOSECOND);