feat(ds): allow to turn monotonic timestamps off for DB

That tells implementation how to assign timestamps to messages. Current
implicit default is now `force_monotonic_timestamps => true`.

apps/emqx_ds_builtin_local/src/emqx_ds_builtin_local_meta.erl

@@ -151,7 +151,6 @@ ensure_monotonic_timestamp(ShardId) ->
 %%================================================================================
 
 -record(s, {}).
--define(timer_update, timer_update).
 
 init([]) ->
     process_flag(trap_exit, true),

apps/emqx_durable_storage/src/emqx_ds.erl

@@ -169,6 +169,13 @@
 -type generic_db_opts() ::
     #{
         backend := atom(),
+        %% Force strictly monotonic message timestamps.
+        %% Default: `true'.
+        %% Messages are assigned unique, strictly monotonically increasing timestamps.
+        %% Those timestamps form a total order per each serialization key.
+        %% If `false' then message timestamps are respected; timestamp, topic and
+        %% serialization key uniquely identify a message.
+        force_monotonic_timestamps => boolean(),
         serialize_by => clientid | topic,
         _ => _
     }.