core: don't start any plugin instance in PluginManager::startProtocolPlugin

As PluginManager::startAllPlugins() have already been called, the instance is
started in createProtocolOwner() which is called by
gUserManager.loadProtocol(). Trying to start it again in startProtocolPlugin()
is a bad idea and makes licq assert.

Fixes #43
Merge branch '1.8-stable'