schema changes to support multi-connections (#372)

internal/db/migrations/postgres/50_session.up.sql

@@ -131,6 +131,12 @@ begin;
     certificate bytea not null,
     -- after this time the connection will be expired, e.g. forcefully terminated
     expiration_time wt_timestamp, -- maybe null
+    -- limit on number of session connections allowed.  default of 0 equals no limit
+    connection_limit int not null default 1
+      check(connection_limit >= 0), 
+    -- connection idle timout in seconds.  default of 0 equals no limit
+    connection_idle_timeout_seconds int not null default 0
+      check(connection_idle_timeout_seconds >= 0),
     -- trust of first use token 
     tofu_token bytea, -- will be null when session is first created
     -- the reason this session ended (null until terminated)
@@ -152,7 +158,7 @@ begin;
     immutable_columns
   before
   update on session
-    for each row execute procedure immutable_columns('public_id', 'certificate', 'expiration_time', 'create_time');
+    for each row execute procedure immutable_columns('public_id', 'certificate', 'expiration_time', 'connection_limit', 'create_time');
   
   create trigger 
     update_version_column