filetransfer: don't allow the queue size to be 0

This inadvertently caused file transfers to hang when used in conjunction
with `http-connections = 0`: the worker checks if the in-progress
items and items we've added to the incoming queue are larger than
the max queue size, which was previously defined to be 5x the value of
`http-connections. Well, as you can probably tell, 5*0=0, and unless the
`.size()` of any of these containers became negative, we would always
wait another 100ms instead of processing the incoming queue.

Because `http-connections = 0` is a special value to signal,
roughly, "as much possible" (mostly depends on how cURL handles
it, but this is probably approximately right), we default to
`std::thread::hardware_concurrency()` (as is done in a couple other
places; specifically, this is what the ThreadPool constructor does for
a value of `0`).

There may be a better fix, but this works for now, and is approximately
right.

Author: cole-h

src/libstore/filetransfer.cc

@@ -731,7 +731,10 @@ struct curlFileTransfer : public FileTransfer
 
     std::thread workerThread;
 
-    const size_t maxQueueSize = fileTransferSettings.httpConnections.get() * 5;
+    const size_t maxQueueSize =
+        (fileTransferSettings.httpConnections.get() ? fileTransferSettings.httpConnections.get()
+                                                    : std::max(1U, std::thread::hardware_concurrency()))
+        * 5;
 
     curlFileTransfer()
         : mt19937(rd())