Replace app env with config

Currently, `stats_db` and `interval` are obtained from vm.args file
via `application:get_env/2`. This is surprising since IOQ already
depends on the config application for most of it's configuration
parameters.

This changes where those configuration values are obtained from
vm.args to .ini files.
diff --git a/src/ioq_server.erl b/src/ioq_server.erl
index c9fd79b..2fce2ef 100644
--- a/src/ioq_server.erl
+++ b/src/ioq_server.erl
@@ -558,20 +558,10 @@
     io_lib:format(Format, [Year, Month, Date, Hour, Minute, Second, Micro]).
 
 get_interval() ->
-    case application:get_env(ioq, stats_interval) of
-    {ok, Interval} when is_integer(Interval) ->
-        Interval;
-    _ ->
-        60000
-    end.
+    config:get_integer("ioq", "stats_interval", 60*1000).
 
 get_stats_dbname() ->
-    case application:get_env(ioq, stats_db) of
-    {ok, DbName} when is_list(DbName) ->
-        DbName;
-    _ ->
-        "stats"
-    end.
+    config:get("ioq", "stats_db", "stats").
 
 split(B) when is_binary(B) ->
     split(B, 0, 0, []);