Since in the function int64_t Throttle::take(int64_t c) in Throttle, if max is zero, then it means the Throttle does not work. So we need to initialize t
he Throttle before we use it. However, in FileJournal, the Throttle for Journal is used before initialized. Therefore, it should be initialized when we c
reate FileJournal instance.

Signed-off-by: Ning Yao <zay11022@gmail.com>