Adds "on connection added" functor to ManagedConnections

Also adds test case for it
Removes debug output
Merge pull request #43 from inetic/MAID-311-get-available-endpoint-issue

Adds "on connection added" functor to ManagedConnections
Adds mutex around std::future::set_value in two tests

This is another way of making sure promise won't get
destroyed by going out of scope before promise::set_value
finishes.

The tests are BEH_API_SimpleSend and FUNC_API_ParallelSend.