TrxUndoRsegsIterator::m_purge_sys: Remove. There is only one purge_sys.

purge_sys_t: Renamed from trx_purge_t. Define a constructor and
destructor. Allocate rseg_iter, purge_queue inline.

purge_sys->trx: Remove. Use purge_sys->sess->trx instead.

purge_sys->view_active: Remove. Access to purge_sys->view is always
protected by purge_sys->latch.

trx_purge_sys_create(): Replaced by purge_sys_t::purge_sys_t().

trx_purge_sys_close(): Replaced by purge_sys_t::~purge_sys_t().