BUG/MINOR: server: Memory leak of proxy.used_server_addr during deinit

BUG/MINOR: server: Memory leak of proxy.used_server_addr during deinit

GitHub Issue #1037 Reported a memory leak in deinit() caused by an
allocation made in sa2str() that was stored in srv_set_addr_desc().

When destroying each server for a proxy in deinit, include freeing the
memory in the key of server->addr_node.

The leak was introduced in commit 92149f9a8 ("MEDIUM: stick-tables: Add
srvkey option to stick-table") which is not in any released version so
no backport is needed.

Cc: Tim Duesterhus <tim@bastelstu.be>