Fix race conditions is case of threaded Iterate() call

[nvoronchev]

For example, in mono/uia2atk project one use two threads:

• the main thread is used to send outgoing DBus-messages;
• a dedicated thread spawned to call Bus.Session.Iterate() into an infinite loop to serve incoming DBus-messages.

This leads to numerous multithreading issues. Current PR is aimed to fix them.