this change is one of the series which drops most of the callers
using SSTable generation as integer. as the generation of SSTable
is but an identifier, we should not use it as an integer out of
generation_type's implementation. so, in this change, instead of
using the helper accepting int, we switch to the one which accepts
generation_type by offering a default paramter, which is a
generation created using 1. this preserves the existing behavior.
we will divert other callers of `reusable_sst(...,
generation_type::int)` in following-up changes in different ways.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
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