Resolve "Add in scan statistics *metadata* reading"

Closes #2050 (closed)

Closes #1768 (closed)

Related #2067 (add event processing to time statistics)

• add sum to scan time statistics
• sort time statistics by sum (before it was sorted by device name)
• time profiling of scan tasks other than the acquisition chain
• time profiling of device wait_reading
• refactor acquisition chain:
  • acquisition object methods _prepare, _start, ... are renamed to acq_prepare, acq_start, ... (they wrap prepare, start, ... which are implemented by the derived classes)
  • add acq_wait_ready and acq_wait_reading with the same logic as the others acq_prepare, acq_start, ...
  • don't pass the stats_dict around, instead have the AcquistionObject hold a reference to the _stats_dict of the AcqusitionChain