(jit-lock-defer-time): New var.

(jit-lock-defer-time): New var.
(jit-lock-defer-timer, jit-lock-buffers): New vars.
(jit-lock-mode): Initialize them.  Cancel the timers more carefully.
(jit-lock-function): Defer fontification if requested.
(jit-lock-stealth-chunk-start): Pay attention to the new non-nil value.
(jit-lock-stealth-fontify): Check the new `jit-lock-defer-multiline'
text property.
(jit-lock-deferred-fontify): New fun.