fix rate limiter to avoid starvation
Summary:
The current implementation of rate limiter has the possibility to introduce resource starvation when change its limit.
This diff aims to fix this problem by consuming request bytes partially.
Test Plan:
```
./rate_limiter_test
[==========] Running 4 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 4 tests from RateLimiterTest
[ RUN ] RateLimiterTest.OverflowRate
[ OK ] RateLimiterTest.OverflowRate (0 ms)
[ RUN ] RateLimiterTest.StartStop
[ OK ] RateLimiterTest.StartStop (0 ms)
[ RUN ] RateLimiterTest.Rate
request size [1 - 1023], limit 10 KB/sec, actual rate: 10.355712 KB/sec, elapsed 2.00 seconds
request size [1 - 1023], limit 20 KB/sec, actual rate: 19.136564 KB/sec, elapsed 2.00 seconds
request size [1 - 2047], limit 20 KB/sec, actual rate: 20.783976 KB/sec, elapsed 2.10 seconds
request size [1 - 2047], limit 40 KB/sec, actual rate: 39.308144 KB/sec, elapsed 2.10 seconds
request size [1 - 4095], limit 40 KB/sec, actual rate: 40.318349 KB/sec, elapsed 2.20 seconds
request size [1 - 4095], limit 80 KB/sec, actual rate: 79.667396 KB/sec, elapsed 2.20 seconds
request size [1 - 8191], limit 80 KB/sec, actual rate: 81.807158 KB/sec, elapsed 2.30 seconds
request size [1 - 8191], limit 160 KB/sec, actual rate: 160.659761 KB/sec, elapsed 2.20 seconds
request size [1 - 16383], limit 160 KB/sec, actual rate: 160.700990 KB/sec, elapsed 3.00 seconds
request size [1 - 16383], limit 320 KB/sec, actual rate: 317.639481 KB/sec, elapsed 2.50 seconds
[ OK ] RateLimiterTest.Rate (22618 ms)
[ RUN ] RateLimiterTest.LimitChangeTest
[COMPLETE] request size 10 KB, new limit 20KB/sec, refill period 1000 ms
[COMPLETE] request size 10 KB, new limit 5KB/sec, refill period 1000 ms
[COMPLETE] request size 20 KB, new limit 40KB/sec, refill period 1000 ms
[COMPLETE] request size 20 KB, new limit 10KB/sec, refill period 1000 ms
[COMPLETE] request size 40 KB, new limit 80KB/sec, refill period 1000 ms
[COMPLETE] request size 40 KB, new limit 20KB/sec, refill period 1000 ms
[COMPLETE] request size 80 KB, new limit 160KB/sec, refill period 1000 ms
[COMPLETE] request size 80 KB, new limit 40KB/sec, refill period 1000 ms
[COMPLETE] request size 160 KB, new limit 320KB/sec, refill period 1000 ms
[COMPLETE] request size 160 KB, new limit 80KB/sec, refill period 1000 ms
[ OK ] RateLimiterTest.LimitChangeTest (5002 ms)
[----------] 4 tests from RateLimiterTest (27620 ms total)
[----------] Global test environment tear-down
[==========] 4 tests from 1 test case ran. (27621 ms total)
[ PASSED ] 4 tests.
```
Reviewers: sdong, IslamAbdelRahman, yiwu, andrewkr
Reviewed By: andrewkr
Subscribers: andrewkr, dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D60207
Aaron Gao