protocol.h: Make enums in GetDataMsg concrete values
protocol.h: Make enums in GetDataMsg concrete values

This concretizes the numbers and adds a comment to make it clear that
these numbers are fixed by the protocol, and may avoid people forgetting
to claim numbers in the future (e.g. issue #8500).

Also gets rid of a weird unused `MSG_TYPE_MAX` in the middle of the
enumeration (thanks @paveljanik for noticing).

gjhiggins/vcoincore,gjhiggins/vcoincore,gjhiggins/vcoincore,gjhiggins/vcoincore,gjhiggins/vcoincore,gjhiggins/vcoincore