Unnamed lockguards don't actually lock and don't produce
warnings.

See merge request !502

So far the code could try to create a new event even if no slots were available.
This was due to the fact that there was not count of free slots.
The consequences where an exception out of range further down the road as the return code of the event creation was on top not checked properly.
This adds a free slot counter in the white board

See merge request !474

See merge request !466

Allow AthMpEvtLoopMgr to call ConcurrencyFlags::setNumProcs()

See merge request !467

pyRoot will not perform the implict conversion from a string to
a string_ref -- so add a helper which does.

- replace union with boost::variant
- replace call_ template with simpler fwd template
- add compose to compose lambdas

This allows/forces DataSvc and TsDataSvc to avoid temporary std::string
objects.

See merge request !493

See merge request !494

See merge request !482

Credits to Hadrien for spotting that possibility.
Also dropped a couple of dynamic_casts that were simply not useful and moved the waiting list from map to unordered_map (credits to Hadrien again)

See merge request !486

See merge request !481

See merge request !492

See merge request !483

- prefer `optional` over `unique_ptr` to make a nullable type,
  and avoid heap-allocation of RootFileHandler and POSIXFileHandler
- better implementation of `select1st`,`select2nd`
- prefer value-semantics implementation of `get_bit` and `set_bit`

- remove virtual function destructor, qualify as final
- implement binary operator as friend functions
- implement less than using std::tie

- re-use Gaudi::Details::ostream_joiner
- avoid heap allocation
- avoid recursive variadic pack expansion

See merge request !479

See merge request !478

See merge request !452

See merge request !411