MINOR: server: Parse sni and pool-conn-name expressions in a dedicated function

MINOR: server: Parse sni and pool-conn-name expressions in a dedicated function

This change is mandatory to fix an issue. The parsing of sni and
pool-conn-name expressions (from string to expression) is now handled in a
dedicated function. This will avoid to duplicate the same code at different
places.

(cherry picked from commit 086a248645c8edca681adba50b34d9970e455a98)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 79a6520cea23728ebcb138612bec1d077fd64cca)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit c2a750108a2a1f0fe78e484bc103416ba879051a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>