Remove the asynchronus signal handler to detect the child exit.

The connection with MySQL server sometimes disconnects in a test
on TravisCI. I guess this is due to the interruption of the asynchronus
signal handler for SIGCHLD, from the following two reasons.
(1) Error message contains "system error: 4"
    (4 may mean EINTR)
(2) The frequency of the error has increased after tests that use
    many child processes and kill them, was added.

This patch prevents the interput by the asynchronus signal handler
and collects children by the dedicated thread and .
Remove the temporary block of SIGCHILD, because the signal handler was removed in the previos commit.