murmurhash is faster than Jenkins and slightly higher quality, so switch to
it for hashing words.

The best timings I got for hashing for data lengths of the following
numbers of 32-bit words, in seconds per 1,000,000,000 hashes, were:

words     murmurhash      Jenkins hash
-----     ----------      ------------
   1           8.4              10.4
   2          10.3              10.3
   3          11.2              10.7
   4          12.6              18.0
   5          13.9              18.3
   6          15.2              18.7

In other words, murmurhash outperforms Jenkins for all input lengths other
than exactly 3 32-bit words (12 bytes).  (It's understandable that Jenkins
would have a best case at 12 bytes, because Jenkins works in 12-byte
chunks.)  Even in the case where Jenkins is faster, it's only by 5%.  On
average within this data set, murmurhash is 15% faster, and for 4-word
input it is 30% faster.

We retain Jenkins for flow_hash_symmetric_l4() and flow_hash_fields(),
which are cases where the hash value is exposed externally.

This commit appears to improve "ovs-benchmark rate" results slightly by
a few hundred connections per second (under 1%), when used with an NVP
controller.

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