MINOR: server: generalize sni expr parsing

MINOR: server: generalize sni expr parsing

Two functions exists for server sni sample expression parsing. This is
confusing so this commit aims at clarifying this.

Functions are renamed with the following identifiers. First function is
named parse_srv_expr() and can be used during parsing. Besides
expression parsing, it has ensure sample fetch validity in the context
of a server line.

Second function is renamed _parse_srv_expr() and is used internally by
parse_srv_expr(). It only implements sample parsing without extra
checks. It is already use for server instantiation derived from
server-template as checks were already performed. Also, it is now used
in http-client code as SNI is a fixed string.

Finally, both functions are generalized to remove any reference to SNI.
This will allow to reuse it to parse other server keywords which use an
expression. This will be the case for the future keyword pool-conn-name.