Summary:
Extend "J=<parallel>" to valgrind_check.
For DBTest, modify the script to run valgrind. For other tests, prefix launch command with valgrind.

Test Plan: Run valgrind_check with J=1 and J>1 and make sure tests run under valgrind. Manually change codes to introduce memory leak and make sure "make watch-log" correctly report it.

Reviewers: yhchiang, yiwu, andrewkr, kradhakrishnan, IslamAbdelRahman

Reviewed By: kradhakrishnan, IslamAbdelRahman

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56727