Support alternate mechanisms to MongoDB

This set of patches:

* Enables ZeroMQ as a message bus (as an alternative to MongoDB)
* Supports in-memory tables for data storage (as an alternative to MongoDB)
* Fixes two small bugs (in RFClient::load_interfaces and FlowTable::GWResolverCB)

A wrinkle is that a C++ BSON data library is needed. The first patch pulls this code from the mongodb cxx driver.

This set is made against the vandervecken branch, as that is where it has been tested.