reactor: eager close of resource fds in the pending destruction list

[ymmt2005]

The file descriptor of a resource in the pending destruction list
may not be closed. This commit closes them when the list is fixed
for the next GC, adding more chance to close file descritors early.

[nojima]

@ymmt2005
I would like to clarify the purpose of this change.

If I am correct, fix_garbage is not called when object GC runs longer than gc_interval because handler->on_master_interval starts object GC if it can, and then reactor_gc_ready returns false. Therefore, I don't think this is a solution to #97.

[ymmt2005]

@nojima
Correct. This is unrelated to #97.
This merely adds more chances to close file descriptors.

[nojima]

@ymmt2005 I got it

[nojima]

Looks good to me

[ymmt2005]

Thank you for your review!