BUG/MINOR: option prefer-last-server must be ignored in some case

when using "option prefer-last-server", we may not always stay on
the same backend if option balance told us otherwise.
For example, backend may change in the following cases:
balance hdr()
balance rdp-cookie
balance source
balance uri
balance url_param

[wt: backport this to 1.7 and 1.6]
1 file changed