AthenaKernel+AthenaPoolCnvSvc: Add support for running multiple new SharedWriter instances in parallel

As discussed in ATEAM-739, currently one needs to explicitly set the port number in order to run multiple instances of the new SharedWriter in parallel. This MR changes this behavior. Now we delegate the port selection to ROOT to avoid collisions.

I'll leave this in Draft mode until @gemmeren gives the go ahead. I locally tested the changes by running four AthenaMP instance in parallel on the same machine and things seem to work as expected. Nonetheless, an independent check/confirmation is, of course, most welcome.