[logback-user] allow configuration of max connections in ReceiverBase

Hi, in the context of remote socket logging the maximum amount of possible socket connections is defined through the max pool size of the context’s executor service. It would be great if the amount of socket connections would be configurable. Further I think the amount of socket connections should be independent from the context's executor service, meaning the ReceiverBase should have it's own executor service. If there is an agreement, I would be happy to provide a pull request for this. Cheers! Sebastian