Make visitCommand return an Object instead of an InvocationStage

Replicated read benchmarks show a significant amount of time spent in BasicInvocationStage.toInvocationStage().

We can make visitCommand return an Object instead of an InvocationStage, and that return value can be cast to an InvocationStage when a callback is needed. We can't call thenApply() etc. directly on an Object, but we can combine invokeNext(ctx, command).thenApply(function) into invokeNextThenApply(ctx, command, function).