BUG/MINOR: proxy: free persist_rules

BUG/MINOR: proxy: free persist_rules

force-persist proxy keyword is converted into a persist_rule, stored in
proxy persist_rules list member. Each new rule is dynamically allocated
during parsing.

This commit fixes the memory leak on deinit due to a missing free on
persist_rules list entries. This is done via deinit_proxy()
modification. Each rule in the list is freed, along with its associated
ACL condition type.

This can be backported to every stable version.