table: snapshot: move pending_snapshots.erase from seal_snapshot

Now that seal_snapshot doesn't need to lookup
the snapshot_manager in pending_snapshots to
get to the file_sets, erasing the snapshot_manager
object can be done in table::snapshot which
also inserted it there.

This will make it easier to get rid of it in a later patch.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

replica/table.cc

@@ -1391,9 +1391,6 @@ table::seal_snapshot(sstring jsondir, std::vector<snapshot_file_set> file_sets)
         });
     }).then([jsondir] {
         return io_check(sync_directory, std::move(jsondir));
-    }).finally([jsondir] {
-        pending_snapshots.erase(jsondir);
-        return make_ready_future<>();
     });
 }
 
@@ -1511,6 +1508,7 @@ future<> table::snapshot(database& db, sstring name) {
             } catch (...) {
                 ex = std::current_exception();
             }
+            pending_snapshots.erase(jsondir);
             snapshot->manifest_write.signal(smp::count);
         }
         tlogger.debug("snapshot {}: waiting for manifest on behalf of shard {}", jsondir, requester);