Fixes DOOCS server shutdown lock-up

Issue:
Server throws when there are no device module variables:

terminate called after throwing an instance of 'std::logic_error'
  what():  ReadAnyGroup has no element with AccessMode::wait_for_new_data.
Aborted (core dumped)

This commit introduces a workaround as proposed by jens:
  void DoocsUpdater::updateLoop(){
       if (_elementsToRead.empty()) {
           return;
       }

This commit is intended to be reverted if the above strategy has side
effects.

- Replace the std::reference_wrapper<TransferElement> with a TransferElementAbstractor.

Temporary roll-back to the periodic update polling instead of the readAny()-based update logic. This is necessary, since the readAny() implementation seems to be too slow for bigger servers like the LLRF server. A proper solution should be found to improve the performance of readAny() for larger number of variables.

- fixed last tests which were not using timeout based checks

changed updater to run in its own thread. Tests still using doocs update, which is just doing an insanely long sleep of 1 second.

changed DoocsFactory to not create unnecessary decorators of the wrong type when auto-creating variables. Test for spectra still failing

Updater is using a vector of callbacks now. Not working as intended because there are different decorators where they should be the same.

- Changes update to readLatest to allow consistent testing