MINOR: http: add option prefer-last-server

MINOR: http: add option prefer-last-server

When the load balancing algorithm in use is not deterministic, and a previous
request was sent to a server to which haproxy still holds a connection, it is
sometimes desirable that subsequent requests on a same session go to the same
server as much as possible. Note that this is different from persistence, as
we only indicate a preference which haproxy tries to apply without any form
of warranty. The real use is for keep-alive connections sent to servers. When
this option is used, haproxy will try to reuse the same connection that is
attached to the server instead of rebalancing to another server, causing a
close of the connection. This can make sense for static file servers. It does
not make much sense to use this in combination with hashing algorithms.