[C++][FlightRPC] UCX server should be able to shed load

[asfimport]

The UCX server from ARROW-15706 will accept connections and put them into a queue to be handled. If they aren't handled quickly enough this can lead to a lot of clients stuck waiting for the server. The server should reject connections if too many pile up so the client can error or retry or connect to a different server. (This is a pitfall of gRPC/Java that we should avoid here.)

Reporter: David Li / @lidavidm

Related issues:

• [C++][FlightRPC] Implement a UCX transport (is related to)

_{Note: This issue was originally created as ARROW-16124. Please see the migration documentation for further details.}