P2P: Do not burden the main thread with the connection monitor

[heifner]

Currently net_plugin launches the connection monitor and keep alive timer on the main thread:

spring/plugins/net_plugin/net_plugin.cpp

Lines 4529 to 4530 in a5f0a7f

	my->ticker();
	my->start_monitors();

These can and should be started on the net_plugin thread pool.