(maint) Fix Connector::monitor_thread_
(maint) Fix Connector::monitor_thread_

Fix wait condition; unlock before returning in case the dtor was called.

puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client