Fix race condition

[shuding]

Make sure mutate, revalidate and trigger won't be ignored because of deduping and race conditions:

+------------------------------------------------------->A
ongoing request                                         stale response
       +-------------------------------------->B
       revalidate                             latest data

It should show B but the result is overridden by A, which is wrong. The sequence of responses should be ordered increasingly, by the start time of the corresponding requests.

Fix #153.