The only user is row level repair: it is replaced with
downgrade_to_v1(make_empty_flat_reader_v2()). The row level reader has
lots of downgrade_to_v1() calls, we will deal with these later all at
once.
Another use is the empty mutation source, this is trivially converted to
use the v2 variant.
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
