Fix for exhausting the Neo4j Server ThreadPool with too many selectors, so that no acceptors are available

* extracted HttpConnectorFactory
* SslConnectorFactory inherits from it to use the same approach
* Jetty makes wrong choices for selectors (numcpu instead of numcpu/2), so we have to provide explicit counts for selectors and acceptors
* Determine number of relevant CPUs by taking the size of the thread pool into account
* Add some other performance related configurations from the Jetty docs
Merge pull request #2199 from jexp/server-threadpool

Fix for exhausting the Neo4j Server ThreadPool with too many selectors, so that no acceptors are available