org-indent: also initialize background buffers, less aggressively

org-indent: also initialize background buffers, less aggressively

* lisp/org-indent.el (org-indent-initialize-agent): when the current
  buffer isn't being watched, resume initialization of other watched
  buffers. In that case, give hand to others idle timers or processes
  more frequently.
(org-indent-agent-active-delay): renamed from
org-indent-agent-process-duration.
(org-indent-agent-passive-delay): new variable.
(org-indent-agent-resume-delay): change value.
(org-indent-initialize-buffer): change argument name.
(org-indent-add-properties): change argument name and type
expected. It must be a time value now.

add