Introduce mutation/ module

Move mutation-related files to a new mutation/ directory. The names
are kept in the global namespace to reduce churn; the names are
unambiguous in any case.

mutation_reader remains in the readers/ module.

mutation_partition_v2.cc was missing from CMakeLists.txt; it's added in this
patch.

This is a step forward towards librarization or modularization of the
source base.

Closes #12788

db/commitlog/commitlog_replayer.cc

@@ -29,7 +29,7 @@
 #include "db/extensions.hh"
 #include "utils/fragmented_temporary_buffer.hh"
 #include "validation.hh"
-#include "mutation_partition_view.hh"
+#include "mutation/mutation_partition_view.hh"
 
 static logging::logger rlogger("commitlog_replayer");