- fix r2076: removed cache.session_id column, removed DELETEs from cache in session_gc

- trust DB server's time when "touching" cache and messages tables

SQL/postgres.initial.sql

@@ -129,14 +129,12 @@ CREATE SEQUENCE cache_ids
 CREATE TABLE "cache" (
     cache_id integer DEFAULT nextval('cache_ids'::text) PRIMARY KEY,
     user_id integer NOT NULL REFERENCES users (user_id) ON DELETE CASCADE ON UPDATE CASCADE,
-    session_id character varying(40) REFERENCES "session" (sess_id) ON DELETE CASCADE ON UPDATE CASCADE,
     cache_key character varying(128) DEFAULT ''::character varying NOT NULL,
     created timestamp with time zone DEFAULT now() NOT NULL,
     data text NOT NULL
 );
 
 CREATE INDEX cache_user_id_idx ON "cache" (user_id, cache_key);
-CREATE INDEX cache_session_id_idx ON "cache" (session_id);
 
 --
 -- Sequence "message_ids"