The sstable_writer_config creation looks simple (just declare
the struct instance) but behind the scenes references storage
and feature services, messes with database config, etc.
This patch teaches the sstables_manager generate the writer
config and makes the rest of the code use it. For future
safety by-hands creation of the sstable_writer_config is
prohibited.
The manager is referenced through table-s and sstable-s, but
two existing sstables_managers live on database object, and
table-s and sstable-s both live shorter than the database,
this reference is save.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>