Fix the timing out flakiness revealed in dataset_map_test.go

[justicezyx]

Frequent test timing out happened in the Travis CI. The error message usually looks like:

cogroup testing start...
panic: test timed out after 1m30s

Find the root cause and eliminate the flakiness.

[justicezyx]

672d457 might already fix the bug (or one of them) that causes timing out.

Will keep monitoring the status of Travis CI build.

[justicezyx]

adef028 fixes a similar issue as 672d457, but the same test timing out still appears.

We need to track down the similar issues as fixed in these 2 commits. And look for other potential causes.

Another difficulty in fixing the time out is that it cannot be reproduced on my local desktop, even with a very large number of repeated runs. I tried once to run the 'go test -race' command for several hours, and there is no timing out.