Convert some old `const char*` to `StringData`.
(came up in a code review)

Provide a `fmt::formatter` and `operator<<` for these so we don't have to call the `${X}ToString` functions much at all. Rename them just `toStringData` and let overloading do the spelling work.

Upgrade msgasserted.
Upgrade NetworkOp from `enum` to `enum class`.

Rename MSGHEADER namespace and MsgData namespace to comply with namespace convention.