Fix a bug in lsm_tree:close/1.

There is a race condition based on the monitor set in a call. We might
get a normal exit from the monitor message deep inside gen_server.
This has to be handled. I've seen this race in my QC tests.

src/lsm_btree.erl

@@ -23,7 +23,9 @@ close(Ref) ->
         gen_server:call(Ref, close)
     catch
         exit:{noproc,_} -> ok;
-        exit:noproc -> ok
+        exit:noproc -> ok;
+        %% Handle the case where the monitor triggers
+        exit:{normal, _} -> ok
     end.