remote_storage: enable Azure connection pooling by default (#10324)

## Problem

Initially we defaulted this to zero to reduce risk. We have now been
using pooling in staging for some time without issues, so let's make it
the default for anyone using this software without setting the config
explicitly.

Closes: https://github.com/neondatabase/cloud/issues/20971

## Summary of changes

- Set Azure blob storage connection pool size to 8 by default

libs/remote_storage/src/config.rs

@@ -115,13 +115,15 @@ fn default_max_keys_per_list_response() -> Option<i32> {
 }
 
 fn default_azure_conn_pool_size() -> usize {
-    // Conservative default: no connection pooling.  At time of writing this is the Azure
-    // SDK's default as well, due to historic reports of hard-to-reproduce issues
+    // By default, the Azure SDK does no connection pooling, due to historic reports of hard-to-reproduce issues
     // (https://github.com/hyperium/hyper/issues/2312)
     //
     // However, using connection pooling is important to avoid exhausting client ports when
     // doing huge numbers of requests (https://github.com/neondatabase/cloud/issues/20971)
-    0
+    //
+    // We therefore enable a modest pool size by default: this may be configured to zero if
+    // issues like the alleged upstream hyper issue appear.
+    8
 }
 
 impl Debug for S3Config {