BUG/MINOR: stick-table: release server_key on any stksess destruction

BUG/MINOR: stick-table: release server_key on any stksess destruction

A stick-table storing server_key holds a reference on a dictionary entry
for each of its entries. This reference is released by stksess_free()
only, while expiry (process_table_expire()), eviction
(stktable_trash_oldest()) and explicit deletion (__stksess_kill(), also
used by "clear table") destroy the session through __stksess_free()
directly. The dictionary entry is then never freed: it stays in
server_key_dict with a refcount of 1 and no owner, and memory grows
linearly with the number of distinct values that went through the table.

Move the release into __stksess_free() so that every destruction path
releases it, and leave stksess_free() as a simple wrapper.

This must be backported to all stable versions.