js: read_options: support on_update (match 478a321d)

This is a bit different than the lua code: on script-opts change it
simply re-applies the conf-file and script-opts to the options object,
and if this results in any changed value at options then on_update is
called with the changelist as argument.

This allows a value to revert back to the conf-file value if the
matching script-opts key had a different value and then got deleted.
It also guarantees to call back whenever the options object is
modified, which the lua code doesn't do (e.g. if the caller changed
a value and the observer changed it back - it won't detect a change).

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