Make Netty leak detection level immutable

From https://github.com/eclipse-vertx/vert.x/blob/6a8aefb6ec84278774d2c03fdae83fdf1deafded/src/main/java/io/vertx/core/impl/VertxImpl.java#L104 it appears that Vertx, if not configured to do things differently, just set the Netty leak detection level to be DISABLED.

Together with https://github.com/netty/netty/pull/13459 this can make such decision evident at JIT level, but loosing the ability to change it at runtime (but with some performance benefits yet to test in JIT mode).