Improve performance when looking for summary restart files

[magnesj]

The functions RimFileSummaryCase::createSummaryReaderInterfaceThreadSafe is not using opm-common reader to find file path of all restart files for a summary case. This is a performance issue, and should be rewritten.

Developer note: Using ESmry::make_esmry_file() supports only a single summary case without restart data. This means that it is required to find the path for all restart files and create a reader for them when data is imported in ESMRY file format.