headers: Refine view_update_generator.hh and around

The initial intent was to reduce the fanout of shared_sstable.hh through
v.u.g.hh -> cql_test_env.hh chain, but it also resulted in some shots
around v.u.g.hh -> database.hh inclusion.

By and large:
- v.u.g.hh doesn't need database.hh
- cql_test_env.hh doesn't need v.u.g.hh (and thus -- the
  shared_sstable.hh) but needs database.hh instead
- few other .cc files need v.u.g.hh directly as they pulled it via
  cql_test_env.hh before
- add forward declarations in few other places

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

Closes #12952

db/view/view_update_generator.cc

@@ -8,6 +8,7 @@
 
 #include <seastar/util/defer.hh>
 #include <boost/range/adaptor/map.hpp>
+#include "replica/database.hh"
 #include "view_update_generator.hh"
 #include "service/priority_manager.hh"
 #include "utils/error_injection.hh"

db/view/view_update_generator.hh

@@ -8,13 +8,20 @@
 
 #pragma once
 
-#include "replica/database.hh"
 #include "sstables/shared_sstable.hh"
 
+#include <seastar/core/metrics_registration.hh>
 #include <seastar/core/abort_source.hh>
 #include <seastar/core/condition-variable.hh>
 #include <seastar/core/semaphore.hh>
 
+using namespace seastar;
+
+namespace replica {
+class database;
+class table;
+}
+
 namespace db::view {
 
 class view_update_generator {