minimal changes required to make OfflineVertexFitter threadsafe

Minimal changes required (but which are not guaranteed to be sufficient! (*)) to make OfflineVertexFitter thread safe:

• Replace Warning and Error with MsgCounter

Some drive-by cleanup:

• Prefer inheritance from extends over virtual inheritance and declareInterface
• Prefer Gaudi::Property over declareProperty
• Prefer range-based loops
• Prefer ServiceHandle and ToolHandle over explicit pointers and properties

Note: OfflineVertexFitter is labelled as "unmaintained" - use at your own risk.

Required for Moore#827 (closed)

(*) but probably this is sufficient...