MINOR: spoe: Use only a global engine-id per agent

MINOR: spoe: Use only a global engine-id per agent

Because the async mode was removed, it is no longer mandatory to announce a
different engine identifiers per thread for a given SPOE agent. This was
used to be sure requests and the corresponding responses are stuck on the
same thread.

So, now, a SPOE agent only announces one engine identifier on all
connections. No changes should be expected for agents.

The related issue is #2502.