Make the asset retrieval concurrency a config switch. The current value

of 30 is still hanging badly on some mono versions. The switch defaults to
30 to preserve current behavior.

bin/OpenSimDefaults.ini

@@ -427,6 +427,10 @@
     ; " (Mozilla Compatible)" to the text where there are problems with a web server
     ;user_agent = "OpenSim LSL (Mozilla Compatible)"
 
+    ; OpenSim can send multiple simultaneous requests for services such as asset
+    ; retrieval. However, some versions of mono appear to hang when there are too
+    ; many simultaneous requests, default is 30 and is currently applied only to assets
+    ;MaxRequestConcurrency = 30
 
 [XMLRPC]
     ; ##