Implement stress option framework.

Implement stress option framework.

Stress options allow developers testing Open vSwitch to trigger behavior
that otherwise would occur only in corner cases.  Developers and testers
can thereby more easily discover bugs that would otherwise manifest only
rarely or nondeterministically.  Stress options may cause surprising
behavior even when they do not actually reveal bugs, so they should only be
enabled as part of testing Open vSwitch.

This commit implements the framework and adds a few example stress options.

This commit started from code written by Andrew Lambeth.

Suggested-by: Henrik Amren <henrik@nicira.com>
CC: Andrew Lambeth <wal@nicira.com>