MINOR: dns: dns_requester structures are now in a memory pool

MINOR: dns: dns_requester structures are now in a memory pool

dns_requester structure can be allocated at run time when servers get
associated to DNS resolution (this happens when SRV records are used in
conjunction with service discovery).
Well, this memory allocation is safer if managed in an HAProxy pool,
furthermore with upcoming HTTP action which can perform DNS resolution
at runtime.

This patch moves the memory management of the dns_requester structure
into its own pool.