MEDIUM: resolvers: add a ref between servers and srv request or used SRV record

MEDIUM: resolvers: add a ref between servers and srv request or used SRV record

This patch add a ref into servers to register them onto the
record answer item used to set their hostnames.

It also adds a head list into 'srvrq' to register servers free
to be affected to a SRV record.

A head of a tree is also added to srvrq to put servers which
present a hotname in server state file. To re-link them fastly
to the matching record as soon an item present the same name.

This results in better performances on SRV record response
parsing.

This is an optimization but it could avoid to trigger the haproxy's
internal wathdog in some circumstances. And for this reason
it should be backported as far we can (2.0 ?)