Add an overall timeout on the client authentication cycle, so that

a hung client or lost connection can't indefinitely block a postmaster
child (not to mention the possibility of deliberate DoS attacks).
Timeout is controlled by new authentication_timeout GUC variable,
which I set to 60 seconds by default ... does that seem reasonable?

src/backend/utils/misc/postgresql.conf.sample

@@ -176,6 +176,7 @@
 #
 #dynamic_library_path = '$libdir'
 #australian_timezones = false
+#authentication_timeout = 60  # min 1, max 600
 #deadlock_timeout = 1000
 #default_transaction_isolation = 'read committed'
 #max_expr_depth = 10000 # min 10