Draft: ComponentAccumulator: implement async merges

Proof-of-concept (not proposing to merge this in its current form) to implement asynchronous merging in ComponentAccumulator, e.g.:

acc = ComponentAccumulator()
acc.async_merge(MyCfg, flags)
acc.async_merge(MyOtherCfg, flags)
...
acc.async_wait()

Changed RecoSteering to apply this for the various reco domains. Results in speedup from 2:00 (91% CPU) minutes to 1:35 (136%) with a process Pool of size 4. This means there is unfortunately not enough parallelism to even fully utilize two cores.

Notes:

• Requires the multiprocess module (available in LCG) which uses dill for object serialization (the default pickle cannot handle the lambas in AthConfigFlags)
• Apart from differences in the order of some property values the final config pkl is the same
• By splitting the configuration into sub-processes we loose potential optimizations from the AccumulatorCache and cache statics are not reported back to the main process
• Cannot be applied "blindly", e.g. post-processing still needs to run last
• Process pool size would need to be made configurable
• Log output is messy

To reproduce one needs to setup the full LCG release:

export LCG_RELEASE_BASE=/cvmfs/sft.cern.ch/lcg/releases
asetup --noLcgReleaseBase Athena,main,latest