* replace generation_type::value() with generation_type::as_int()
* drop generation_value()
because we will switch over to UUID based generation identifier, the member
function or the free function generation_value() cannot fulfill the needs
anymore. so, in this change, they are consolidated and are replaced by
"as_int()", whose name is more specific, and will also work and won't be
misleading even after switching to UUID based generation identifier. as
`value()` would be confusing by then: it could be an integer or a UUID.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
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