Defer setting the short timeout sec

There is a situation that the check requests are enormously issued. When there
are few tasks left and one slave already tried them, the slave repeatedly
requests a task and issues check requests. The example sequence is like below:

1. Slave1 tries to dequeues a task, and there is no tasks left.
2. Slave1 issues a check request, and it sets the long timeout sec to wait for
   the arbiter's judge.
3. Arbiter enqueues a task, which Slave1 already has already tried.
4. Slave1 dequeues a task, and since the slave has been already tried the task,
   it pushes back the task. It sets the short timeout sec as usual.
5. Slave2 dequeues a task.
6. Slave1 tries to dequeues a task, and there is no tasks left.
7. Back to 2.

It is the intended behavior, except that the timeout speed in the step 6 is
faster than expected. This is because, in the step 4, it sets the short timeout
time. This commit defers setting the timeout sec.
Merge pull request #19 from cookpad/suppress_check_request

Defer setting the short timeout sec