This was the last abuse of trx_sys.mutex, which is now exclusively
protecting trx_sys.trx_list.

This global acquisition was also potential scalability bottleneck for
oltp_read_write benchmark. Although it didn't expose itself as such due
to larger scalability issues.

Replaced trx_sys.mutex based synchronisation between ReadView creator
thread and purge coordinator thread performing latest view clone with
ReadView::m_mutex.

It also allowed to simplify tri-state view m_state down to boolean
m_open flag.

For performance reasons trx->read_view.close() is left as atomic relaxed
store, so that we don't have to waste resources for otherwise meaningless
mutex acquisition.