Schema backwards compatability

The replrtq_srcqueuelimit was removed, as the queue is now an overflow queue and should have its limit configured via replrtq_overflow_limit.

However, removing the old config item altogether will result in upgrades not being possible without changes to configuration files - so the old option is instead retained as a hidden config item to be ignored.

priv/riak_kv.schema

@@ -1453,3 +1453,15 @@
     {datatype, integer},
     {default, 5}
 ]}.
+
+
+%% @doc Limit the size of replication queues (for a queue and priority, i.e.
+%% each priority on each queue will have this as the limit)
+%% This option will be ignored from Release 3.0.10, as the queue has been
+%% converted to an overflow queue as part of this release.  Instead use
+%% `replrtq_overflow_limit` to control the queue size, including on-disk size.
+{mapping, "replrtq_srcqueuelimit", "riak_kv.replrtq_srcqueuelimit", [
+  {datatype, integer},
+  {default, 300000},
+  hidden
+]}.