Add read_concurrency option to mem3_shards table

This table sees a great deal of activity from various subsystems -
turning on read_concurrency should be a win.

COUCHDB-2984
diff --git a/src/mem3_shards.erl b/src/mem3_shards.erl
index 224673f..8a1bb54 100644
--- a/src/mem3_shards.erl
+++ b/src/mem3_shards.erl
@@ -181,7 +181,13 @@
     end).
 
 init([]) ->
-    ets:new(?SHARDS, [bag, protected, named_table, {keypos,#shard.dbname}]),
+    ets:new(?SHARDS, [
+        bag,
+        protected,
+        named_table,
+        {keypos,#shard.dbname},
+        {read_concurrency, true}
+    ]),
     ets:new(?DBS, [set, protected, named_table]),
     ets:new(?ATIMES, [ordered_set, protected, named_table]),
     ok = config:listen_for_changes(?MODULE, nil),