Switch to using git submodules for dependencies

Git submodules can be thought of as symlinks to a particular commit of another repository. Fortunately, the mongo-c-driver project appears to have good hygiene when it comes to tagging releases, so I was able to quickly find v0.6 and switch to using a submodule locked to that version.

Using a submodule instead of copying the code reduces the temptation to "fix" bugs in our dependencies without contacting upstream (diverging in the process) and makes updating to a newer version _much_ easier.