mem: Convert Request static const flags to enums

mem: Convert Request static const flags to enums

This patch fixes an issue which is very wide spread in the codebase,
causing sporadic linking failures. The issue is that we declare static
const class variables in the header, without any definition (as part
of a source file). In most cases the compiler propagates the value and
we have no issues. However, especially for less optimising builds such
as debug, we get sporadic linking failures due to undefined
references.

This patch fixes the Request class, by turning the static const flags
and master IDs into C++11 typed enums.