MongoDB: Clean up m_log on delete.
MongoDB: Make the connection object a pointer.
MongoDB: Init collection names through string concatenation rather than stringstream.

I really don't know what I was thinking when I did it this way.
CMake: Fix Boost dependency resolution inconsistency in CMakeLists.txt.
MongoDB: Use connection strings instead of multiple config directives.

This allows for compatibility with replica-set MongoDB backends and simplifies configuration.

N.B. this breaks compatibility with all variants of MongoDB's C++ driver except for 'legacy'