When cloning oldest view, don't copy ReadView::m_creator_trx_id.
It means that the owner thread is now allowed to access this member
without trx_sys.mutex protection.

To achieve this we have to keep ReadView::m_creator_trx_id in
ReadView::m_ids. This is required to not let purge thread process
records owned by transaction associated with oldest view.

It is only required if trsanction entered read-write mode before it's
view was created.

If transaction entered read-write mode after it's view was created
(trx_set_rw_mode()), purge thread won't be allowed to touch it because
m_low_limit_id >= m_creator_trx_id holds. Thus we don't have to add
this transaction id to ReadView::m_ids.

Cleanups:

ReadView::ids_t: don't seem to make any sense, just complicate matters.

ReadView::copy_trx_ids(): doesn't make sense anymore, integrated into
caller.

ReadView::copy_complete(): not needed anymore.

ReadView copy constructores: don't seem to make any sense.

trx_purge_truncate_history(): removed view argument, access
purge_sys->view directly instead.