Move read_life_signal to Pool.
Remove useless children_num.
Remove useless global finished_children_num.
Move write_child_pipe to Pool class.
Change -f to use main as default.
Rename finished_children_num.
Change the rfd to a object property.
print the traceback when the process exits with exception.
Add UT for Pool.
Add heartbeat handling logic.
Fix the event loop bug.

Originally, Pool use the global event loop, which will cause problem if
get two Pool instances.
Test for children after run function.
assert EOF.