flat_mutation_reader: Split readers by file and remove unnecessary includes.

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

db/view/build_progress_virtual_reader.hh

@@ -10,7 +10,7 @@
 #include "db/system_keyspace.hh"
 #include "db/timeout_clock.hh"
 #include "dht/i_partitioner.hh"
-#include "flat_mutation_reader_v2.hh"
+#include "readers/flat_mutation_reader_v2.hh"
 #include "mutation_fragment.hh"
 #include "mutation_reader.hh"
 #include "query-request.hh"

db/view/view.cc

@@ -56,6 +56,7 @@
 #include "utils/exponential_backoff_retry.hh"
 #include "utils/fb_utilities.hh"
 #include "query-result-writer.hh"
+#include "readers/from_fragments.hh"
 
 using namespace std::chrono_literals;
 

db/view/view.hh

@@ -12,7 +12,7 @@
 #include "gc_clock.hh"
 #include "query-request.hh"
 #include "schema_fwd.hh"
-#include "flat_mutation_reader.hh"
+#include "readers/flat_mutation_reader.hh"
 #include "frozen_mutation.hh"
 
 class frozen_mutation_and_schema;

db/view/view_updating_consumer.hh

@@ -19,6 +19,7 @@
 #include "mutation_rebuilder.hh"
 
 class evictable_reader_handle;
+class evictable_reader_handle_v2;
 
 namespace db::view {