More Uses of Unsafe Double-Checked Locking Pattern

[git-afsantos]

The reasoning is the same as per my previous issue, #770. This code pattern is unsafe, and now that recent releases compile under C++11, there is no reason for it to be used. I just found three more occurrences of this unsafe pattern in the code.

1. xmlrpc_manager.cpp
2. poll_manager.cpp
3. connection_manager.cpp