AthenaCommon+AthenaConfiguration: speedup gatherProps

The property serialization in `gatherProps` for jobs with many sequences
(e.g. trigger) was entirely dominated by the recursive sequence
traversal.

Replace the `flatSequencers` by an iterator. This saves memory and
moreover gives direct access to the sequences/algorithms without having
to find them a second time via the name. Also remove the `algsCollection`
feature which does not seem to serve any purpose.

Replace the usage of `flatSequencers` in `getEventAlgo` and
`gatherProps` by the new iterator. Also streamline the treatment of the
"Members" property of sequences.

For a trigger job this reduces the property serialization from 30 to 10
seconds. No changes observed in the final property content. The only
exception are algorithms that are not contained in any sequence. But
this should never happen and is likely a bug in the configuration (see
ATR-28887).