hash: Introduce an implementation of murmurhash.

hash: Introduce an implementation of murmurhash.

Murmurhash is generally superior to the Jenkins lookup3 hash according to
the available figures.  Perhaps we should generally replace our current
hashes by murmurhash.

For now, I'm introducing a parallel implementation to allow it to be used
in cases where an incremental one-word-at-a-time hash is desirable.  The
first user will be added in an upcoming commit.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>