MEDIUM: proxy: take the defsrv out of the struct proxy

MEDIUM: proxy: take the defsrv out of the struct proxy

The server struct has gone huge over time (~3.8kB), and having a copy
of it in the defsrv section of the struct proxy costs a lot of RAM,
that is not needed anymore at run time.

This patch replaces this struct with a dynamically allocated one. The
field is allocated and initialized during alloc_new_proxy() and is
freed when the proxy is destroyed for now. But the goal will be to
support freeing it after parsing the section.