Raise MIN_DL_PER_REQUEST to 32

This change should improve overhead for downloading small numbers of
descriptors and microdescriptors by improving compression
performance and lowering directory request overhead.

Closes ticket 23220.

changes/bug23220

@@ -0,0 +1,7 @@
+  o Minor features (client directory bandwidth tuning):
+
+    - When downloading (micro)descriptors, don't split the list of
+      descriptors into multiple requests unless there are at least 32
+      descriptors that we want. Previously, we split at 4, not 32, which
+      could lead to significant overhead in HTTP request size and
+      degradation in compression performance. Closes ticket 23220.