Move pool dispose() before os.fork

Move pool dispose() before os.fork

Currently pool dispose() is done after os.fork, but this will
produce shared DB connections in child processes which may lead
to DB errors.

Move pool dispose() before os.fork. This will remove all existing
connections in the parent process and child processes will create
their own new ones.

(cherry-picked from 88e499d1c10eaae59546d9f16c9c9c262766de84)

Change-Id: Ie36417a64f0eb39b53dad61517f834aec37bacfb
Closes-Bug: 1458718