Adds some checks about async.

There are modules that will not work with prosody 0.10 as they depend on util.async. Adds a safeguard and print error about it in the logs.
And others that just do not work because of the muc module API that they use.

resources/prosody-plugins/mod_muc_poltergeist.lua

 local neturl = require "net.url";
 local parse = neturl.parseQuery;
 local poltergeist = module:require "poltergeist";
+
+local have_async = pcall(require, "util.async");
+if not have_async then
+    module:log("error", "requires a version of Prosody with util.async");
+    return;
+end
+
 local wrap_async_run = module:require "util".wrap_async_run;
 
 -- Options

resources/prosody-plugins/mod_muc_size.lua

 local json = require "util.json";
 local iterators = require "util.iterators";
 local array = require"util.array";
+
+local have_async = pcall(require, "util.async");
+if not have_async then
+    module:log("error", "requires a version of Prosody with util.async");
+    return;
+end
+
 local wrap_async_run = module:require "util".wrap_async_run;
 
 local tostring = tostring;

resources/prosody-plugins/mod_speakerstats_component.lua

 local socket = require "socket";
 local json = require "util.json";
 
+-- we use async to detect Prosody 0.10 and earlier
+local have_async = pcall(require, "util.async");
+if not have_async then
+    module:log("warn", "speaker stats will not work with Prosody version 0.10 or less.");
+    return;
+end
+
 local muc_component_host = module:get_option_string("muc_component");
 if muc_component_host == nil then
     log("error", "No muc_component specified. No muc to operate on!");
     prosody.events.add_handler("host-activated", process_host);
 else
     process_host(muc_component_host);
-end
+end

resources/prosody-plugins/util.lib.lua

 local jid = require "util.jid";
-local runner, waiter = require "util.async".runner, require "util.async".waiter;
-
+local have_async, async = pcall(require, "util.async");
 local muc_domain_prefix
     = module:get_option_string("muc_mapper_domain_prefix", "conference");
 
 
 
 function wrap_async_run(event,handler)
+    local have_async = pcall(require, "util.async");
+    if not have_async then
+        module:log("error", "requires a version of Prosody with util.async");
+        return nil;
+    end
+
+    local runner = async.runner;
     -- Grab a local response so that we can send the http response when
     -- the handler is done.
     local response = event.response;