m_config_core: fix forced option notification with m_config_cacheHEADmaster

bc28f7693d095758275448410aaa91255482857a originally added this, but the
implementation isn't correct and causes excessive notifications when
writing to other options which may have bad behavior in some some
circumstances. Fix this by reworking the implementation for force
options so that the timestamps of the option update compared instead.
Whenever an option gets changed, the internal timestamp in the cache is
always incremented. For a special force option, we can save this
timestamp internally as well.

Because cache_check_update is always checked before potentially sending
an option notification, we know that if the internal timestamp is equal
to the timestamp saved by a force update option, it must have been
previously written. Thus, the notification can be sent. This lets
options like geometry work repeatedly but without constantly sending
notifications. Fixes #13954.

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