Replace definition of value_sett::object_map_dt

[romainbrenguier]

Instead of defining a new class with the same interface as std::map and
which use std::map internally, we use using and define outside the
static field blank which is the only thing added to std::map.

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[smowton]

s/blank/empty, otherwise lgtm

[tautschnig]

clang-format isn't happy and +1 on using empty_object_map instead of blank_object_map, otherwise looks good, another nice piece of cleanup!