Add Lwan tests.

This patch adds Lwan web server alongside a benchmark that implements
all but one (database updates) tests.
I am not opposed to supporting SQLite as a database permutation type, but it *is* exceptional versus the others in that it does not incur any network latency to speak with a database server.  I know that it has been requested before, but to my memory it has not yet been implemented and submitted as a PR.  My inclination is to accept this and trust that consumers of the results will understand that SQLite is an exceptional test case.

My only hesitation would simply be that we should communicate that we're accepting PRs that use SQLite now.  Since others have asked for this and similar, we should make it clear that it's acceptable.

If possible, I would *strongly* request an LWAN test that uses a traditional database as well, allowing consumers of the data to draw reasonable comparisons to the majority of database tests.