The purpose of this patch is to split the actions of writing sstable and
sealing it. As long as the sstable is unsealed it is considered
incomplete and is going to be removed on reboot.
Such functionality is needed in order to defer visibility of sstables
created during streaming until the streaming is complete.
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>