sstables: allow read_toc to be called more than once
sstables: allow read_toc to be called more than once

We do that by bailing immediately if we detect that the components
map is already populated. This allow us to call read_toc() earlier
if we need to - for instance, to inquire about the existence of the
Summary - without the need to re-read the components again later.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

scylladb/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,kjniemi/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,kjniemi/scylla,duarten/scylla,duarten/scylla