MEDIUM: backend: Enhance hash-type directive with an algorithm options

MEDIUM: backend: Enhance hash-type directive with an algorithm options

Summary:
In testing at tumblr, we found that using djb2 hashing instead of the
default sdbm hashing resulted is better workload distribution to our backends.

This commit implements a change, that allows the user to specify the hash
function they want to use. It does not limit itself to consistent hashing
scenarios.

The supported hash functions are sdbm (default), and djb2.

For a discussion of the feature and analysis, see mailing list thread
"Consistent hashing alternative to sdbm" :

      http://marc.info/?l=haproxy&m=138213693909219

Note: This change does NOT make changes to new features, for instance,
applying an avalance hashing always being performed before applying
consistent hashing.