Fixes a multithreading bug in the ruby rpc_server

This fixes the current ruby server interop test flakiness.

- The interop tests recently changed so that clients access servers in
parallel, meaning that the interop servers handle multiple rpcs at once,
which is how this bug became visible

- since this change, tests run against the ruby interop server have
failed sporadically

- the problem was that the block in #loop_handle_server_calls that is
  passed to the thread pool referenced a var in an enclosing block,
  which resulted in requests being processed by the wrong handler

- this fix ensures that the block to be run on the thread pool thread
  does not have any references to vars in the enclosing block

src/ruby/lib/grpc/generic/rpc_server.rb

@@ -418,11 +418,11 @@ module GRPC
           an_rpc = @server.request_call(@cq, loop_tag, INFINITE_FUTURE)
           break if (!an_rpc.nil?) && an_rpc.call.nil?
 
-          c = new_active_server_call(an_rpc)
+          active_call = new_active_server_call(an_rpc)
-          unless c.nil?
+          unless active_call.nil?
-            mth = an_rpc.method.to_sym
+            @pool.schedule(active_call) do |ac|
-            @pool.schedule(c) do |call|
+              c, mth = ac
-              rpc_descs[mth].run_server_method(call, rpc_handlers[mth])
+              rpc_descs[mth].run_server_method(c, rpc_handlers[mth])
             end
           end
         rescue Core::CallError, RuntimeError => e
@@ -442,6 +442,7 @@ module GRPC
       # allow the metadata to be accessed from the call
       handle_call_tag = Object.new
       an_rpc.call.metadata = an_rpc.metadata  # attaches md to call for handlers
+      GRPC.logger.debug("call md is #{an_rpc.metadata}")
       connect_md = nil
       unless @connect_md_proc.nil?
         connect_md = @connect_md_proc.call(an_rpc.method, an_rpc.metadata)
@@ -454,9 +455,11 @@ module GRPC
       # Create the ActiveCall
       GRPC.logger.info("deadline is #{an_rpc.deadline}; (now=#{Time.now})")
       rpc_desc = rpc_descs[an_rpc.method.to_sym]
-      ActiveCall.new(an_rpc.call, @cq,
+      c = ActiveCall.new(an_rpc.call, @cq,
-                     rpc_desc.marshal_proc, rpc_desc.unmarshal_proc(:input),
+                         rpc_desc.marshal_proc, rpc_desc.unmarshal_proc(:input),
-                     an_rpc.deadline)
+                         an_rpc.deadline)
+      mth = an_rpc.method.to_sym
+      [c, mth]
     end
 
     protected