Add namespace to FDB actor files

This issue is the umbrella issue tracking the effort to add namespace to FDB code base. Idea is that with namespace in place, code would be much easier to read and understand at the call site. The coupling between different parts of the system would be easier to be seen and understood. Also this can help with potential symbol collisions.

This is an umbrella issue for adding namespaces across the FoundationDB actor codebase, but it names no files, tests, or specific entry points. First identify the affected actor files and any existing namespace conventions, then define the scope before changing code. Done means the agreed actor code uses namespaces consistently and the project still builds and tests successfully.

Written by the indexing model from the issue text.