vinyl: simplify index hash building during vylog recovery

For the sake of recovery, we keep all vy_lsm_recovery_info objects in
vy_recovery::index_id_hash, which maps space_id/index_id to the latest
index incarnation. We build this hash while loading records from vylog,
which is rather difficult for understanding, because an LSM tree may
have a counterpart created by incomplete ALTER (see ->prepared) so we
have to link and unlink those objects very carefully. Let's simplify the
procedure by postponing hash building until we have loaded all records.