sstables: coroutinize parse functions
sstables: coroutinize parse functions

Simplify the implementation using coroutines.
This also has the potential to coalesce multiple
allocations into one.

test: unit(dev)

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20211222103126.1819289-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla