* proxyd: Limit the number of concurrent RPCs to backends

We add a new config `max_concurrent_rpcs` under the server section to
prevent too many RPC requests, (and, more loosely, the number of connections),
from being sent upstream. Requests will block, in a FIFO fashion, until the
number of in-flight RPCs is under the limit.

* fix typo
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>