HTTPCLIENT-1493: Conditional stale connection checking of pooled connections

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1599497 13f79535-47bb-0310-9956-ffa450edef68
HTTPCLIENT-1504: do not allow zero total max and max per route settings (also consistent with the behaviour of AbstractNIOConnPool)

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1599533 13f79535-47bb-0310-9956-ffa450edef68
Post HTTPCLIENT-1504 code improvement (suggested by Sebb)

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1599971 13f79535-47bb-0310-9956-ffa450edef68