std::function<> requires the functor it wraps to be copyable, which is
an unnecessarily strict requirement. To relax this, we use
noncopyable_function<> instead. Since the former seems to lack some
disambiguation magic of the latter, we add `_v1` and `_v2` postfixes to
manually disambiguate.
The flat_mutation_reader files were conflated and contained multiple
readers, which were not strictly necessary. Splitting optimizes both
iterative compilation times, as touching rarely used readers doesn't
recompile large chunks of codebase. Total compilation times are also
improved, as the size of flat_mutation_reader.hh and
flat_mutation_reader_v2.hh have been reduced and those files are
included by many file in the codebase.
With changes
real 29m14.051s
user 168m39.071s
sys 5m13.443s
Without changes
real 30m36.203s
user 175m43.354s
sys 5m26.376s
Closes#10194
