Fix issue with inconsistent GenericMap output (#297)

* Fix issue with inconsistent GenericMap output

The FLP GenericMap provided by the agent differs in types depending on
the exporter being used, especially with DirectFLP each records size is
smaller than when used with protobuf, due to protobuf using uint32
almost everywhere. This is not a problem in most cases as FLP generally
converts that in JSON, but it breaks the FLP IPFIX export which expects
strict types.

This commit brings more consistency between the FLP-direct mode and the
protobuf mode, by sharing the same GenericMap encoding function. Which
means the protobuf decode is now done in two steps: first
proto-to-flow.Record, then flow.Record-to-GenericMap.

* fix proto decode test