smp-server: limit concurrency in proxy command processing (#1162)

* smp: put client proxy command processing threads under a shared semaphore

* add LIMITS.max_proc_threads to server config

* rename to PROXY.client_concurrency

* retry on strictly greater than max concurrency

* set default to 16

* rename

* fix limit

---------

Co-authored-by: Evgeny Poberezkin <evgeny@poberezkin.com>

tests/SMPClient.hs

@@ -119,7 +119,8 @@ cfg =
       transportConfig = defaultTransportServerConfig {Server.alpn = Just supportedSMPHandshakes},
       controlPort = Nothing,
       smpAgentCfg = defaultSMPClientAgentConfig,
-      allowSMPProxy = False
+      allowSMPProxy = False,
+      serverClientConcurrency = 2
     }
 
 cfgV7 :: ServerConfig