smp-server: enforce command concurrency limit (#1944)

* smp-server: hold command slot until completion

* smp-server: command slot wait timeout and stats

* smp-server: clarify command slot comments

* smp-server: simplify command slot fork

* smp-server: limit direct RSLV per client

* smp-server: distinct error for RSLV slot timeout

* smp-server: client command slot wait timeout

* smp-server: reply to forwarded RSLV slot timeout

* smp-server: raise default concurrency limits

* smp-server: simplify command slot wait

tests/SMPClient.hs

@@ -278,6 +278,7 @@ cfgMS msType = withStoreCfg (testServerStoreConfig msType) $ \serverStoreCfg ->
       allowSMPProxy = False,
       serverClientConcurrency = 2,
       serverResolverConcurrency = defaultNameResolverConcurrency,
+      maxClientCmdWait = defaultMaxClientCmdWait,
       namesConfig = Nothing,
       information = Nothing,
       startOptions = defaultStartOptions