Merge 'sstables: a bunch of refactors' from Kamil Braun

1. sstables: move `sstable_set` implementations to a separate module

    All the implementations were kept in sstables/compaction_strategy.cc
    which is quite large even without them. `sstable_set` already had its
    own header file, now it gets its own implementation file.

    The declarations of implementation classes and interfaces (`sstable_set_impl`,
    `bag_sstable_set`, and so on) were also exposed in a header file,
    sstable_set_impl.hh, for the purposes of potential unit testing.

2. mutation_reader: move `mutation_reader::forwarding` to flat_mutation_reader.hh

    Files which need this definition won't have to include
    mutation_reader.hh, only flat_mutation_reader.hh (so the inclusions are
    in total smaller; mutation_reader.hh includes flat_mutation_reader.hh).

3. sstables: move sstable reader creation functions to `sstable_set`

    Lower level functions such as `create_single_key_sstable_reader`
    were made methods of `sstable_set`.

    The motivation is that each concrete sstable_set
    may decide to use a better sstable reading algorithm specific to the
    data structures used by this sstable_set. For this it needs to access
    the set's internals.

    A nice side effect is that we moved some code out of table.cc
    and database.hh which are huge files.

4. sstables: pass `ring_position` to `create_single_key_sstable_reader`

    instead of `partition_range`.

    It would be best to pass `partition_key` or `decorated_key` here.
    However, the implementation of this function needs a `partition_range`
    to pass into `sstable_set::select`, and `partition_range` must be
    constructed from `ring_position`s. We could create the `ring_position`
    internally from the key but that would involve a copy which we want to
    avoid.

5. sstable_set: refactor `filter_sstable_for_reader_by_pk`

    Introduce a `make_pk_filter` function, which given a ring position,
    returns a boolean function (a filter) that given a sstable, tells
    whether the sstable may contain rows with the given position.

    The logic has been extracted from `filter_sstable_for_reader_by_pk`.

Split from #7437.

Closes #7655

* github.com:scylladb/scylla:
  sstable_set: refactor filter_sstable_for_reader_by_pk
  sstables: pass ring_position to create_single_key_sstable_reader
  sstables: move sstable reader creation functions to `sstable_set`
  mutation_reader: move mutation_reader::forwarding to flat_mutation_reader.hh
  sstables: move sstable_set implementations to a separate module

db/view/view.cc

@@ -1438,10 +1438,9 @@ view_builder::build_step& view_builder::get_or_create_build_step(utils::UUID bas
 void view_builder::initialize_reader_at_current_token(build_step& step) {
     step.pslice = make_partition_slice(*step.base->schema());
     step.prange = dht::partition_range(dht::ring_position::starting_at(step.current_token()), dht::ring_position::max());
-    step.reader = make_local_shard_sstable_reader(
+    step.reader = step.base->get_sstable_set().make_local_shard_sstable_reader(
             step.base->schema(),
             _permit,
-            make_lw_shared<sstables::sstable_set>(step.base->get_sstable_set()),
             step.prange,
             step.pslice,
             default_priority_class(),

db/view/view_builder.hh

@@ -28,7 +28,6 @@
 #include "query-request.hh"
 #include "service/migration_listener.hh"
 #include "service/migration_manager.hh"
-#include "sstables/sstable_set.hh"
 #include "utils/exponential_backoff_retry.hh"
 #include "utils/serialized_action.hh"
 #include "utils/UUID.hh"

db/view/view_update_generator.cc

@@ -82,7 +82,7 @@ future<> view_update_generator::start() {
                                 tracing::trace_state_ptr ts,
                                 streamed_mutation::forwarding fwd_ms,
                                 mutation_reader::forwarding fwd_mr) {
-                        return ::make_restricted_range_sstable_reader(s, std::move(permit), std::move(ssts), pr, ps, pc, std::move(ts), fwd_ms, fwd_mr);
+                        return make_restricted_range_sstable_reader(std::move(ssts), s, std::move(permit), pr, ps, pc, std::move(ts), fwd_ms, fwd_mr);
                     });
                     auto [staging_sstable_reader, staging_sstable_reader_handle] = make_manually_paused_evictable_reader(
                             std::move(ms),