The backward compatibility code is very expensive, as it triggers
a huge number of QSetting object creation and destruction.
We only need to perform this for settings which have changed,
so add API to flag changed settings and only perform backward
migration of changed settings.
This dramatically improves QgsApplication shutdown time (cuts
1.5 seconds off shutdown on a release build!!)
This adds a mechanism where flushes of QgsSettings to the underlying
ini storage file can be temporarily suspended. It is intended for
code paths where many settings entries are consecutively read/written,
to avoid the very expensive cost of constructing and destructing
multiple QgsSettings objects for each in turn.
When QgsSettings::holdFlush() is called, then a temporary thread local
QgsSettings object will be created and ALL access to settings entries
will use this same object (preventing flushing of it to ini files).
An accompanying QgsSettings::releaseFlush() call MUST be made from
the same thread to destroy the thread local QgsSettings, flush it
to disk, and resume normal operation.
This helps avoid the VERY costly backward migration of settings,
and cuts the run time for qgis_process commands like `qgis_process list`
by at least half (and considerably more in common setups).
