Make Felix Resolver Threads configurable

It was requested in https://issues.apache.org/jira/browse/FELIX-5247 to make the resolver threads configurable to avoid creating many threads by karaf.
It turns out Felix already has that feature, but Karaf is not using it:

https://github.com/apache/karaf/blob/karaf-4.0.x/features/core/src/main/java/org/apache/karaf/features/internal/osgi/Activator.java#L119

Resolver resolver = new ResolverImpl(new Logger(Logger.LOG_INFO));

I propose to configure and supply an Executor with the desired threads. As additional up, this prevents situations where there are unnecessary threads created.