the invalid sstable id is the NULL of a sstable identifier. with
this concept, it would be a lot simpler to find/track the greatest
generation. the complexity is hidden in the generation_type, which
compares the a) integer-based identifiers b) uuid-based identifiers
c) invalid identitifer in different ways.
so, in this change
* the default constructor generation_type is
now public.
* we don't check for empty generation anymore when loading
SSTables or enumerating them.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
2d265e860d