HSEARCH-1249 Serialization protocol version being logged without enough ...

...context

This was being requested by both Ales and Randall (although at low priority), but I only expected to have to improve an existing logging message;
but we reuse SerializationProvider instances across different indexes, so instead I ended up breaking a "very internal" SPI.

I'd not expect people to implement this one yet, but WDYT?

Alternatives to merging are:
- to postpone this to 4.3
- to rely on #toString instead of adding an explicit method to desribe the service implementation

https://hibernate.onjira.com/browse/HSEARCH-1249