commitlog: Fix use-after-free on mutation object during replay

The mutation object may be freed prematurely during commitlog replay
in the schema upgrading path. We will hit the problem if the memtable
is full and apply_in_memory() needs to defer.

This will typically manifest as a segfault.

Fixes #6953

Introduced in 79935df

Tests:
  - manual using scylla binary. Reproduced the problem then verified the fix makes it go away

Message-Id: <1596044010-27296-1-git-send-email-tgrabiec@scylladb.com>

db/commitlog/commitlog_replayer.cc

@@ -304,7 +304,7 @@ future<> db::commitlog_replayer::impl::process(stats* s, commitlog::buffer_and_r
                 mutation m(cf.schema(), fm.decorated_key(*cf.schema()));
                 converting_mutation_partition_applier v(cm, *cf.schema(), m.partition());
                 fm.partition().accept(cm, v);
-                return do_with(std::move(m), [&db, &cf] (mutation m) {
+                return do_with(std::move(m), [&db, &cf] (const mutation& m) {
                     return db.apply_in_memory(m, cf, db::rp_handle(), db::no_timeout);
                 });
             } else {