ARROW-9168: [C++][Flight] Don't share TCP connection among clients
ARROW-9168: [C++][Flight] Don't share TCP connection among clients

Flight benchmark performs worse when working threads increases. It can
be improved by setting gRPC option GRPC_ARG_USE_LOCAL_SUBCHANNEL_POOL
to make each client creating its own TCP connection to server, without
sharing one connection.

Closes #7476 from cyb70289/flight

Authored-by: Yibo Cai <3ef275458f780e09a5423d1ca1f2a95028f93f71@gmail.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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