Prevent repeated enum (or other) sends accross the wire

[gordonwatts]

You see errors like this:

INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum CaloSample already defined in namespace CaloSampling
INFO:func_adl_xAOD.common.cpp_types:Enum SummaryType already defined in namespace xAOD
INFO:func_adl_xAOD.common.cpp_types:Enum SummaryType already defined in namespace xAOD
INFO:func_adl_xAOD.common.cpp_types:Enum SummaryType already defined in namespace xAOD
INFO:func_adl_xAOD.common.cpp_types:Enum SummaryType already defined in namespace xAOD
INFO:func_adl_xAOD.common.cpp_types:Enum SummaryType already defined in namespace xAOD

This is because the same MetaData is sent many times. Might be worth having a pruning algorithm that removes some of them! Less wire traffic, database usage, etc.