libstore: set connect-timeout to 5

Right now, Lix waits for quite a while five times[1] if a substituter is
down. If the substituter is not reachable for that long, it's probably
down or so slow that it doesn't make sense to download from it. Also,
most people would abort earlier assuming that Lix just behaves weird in
that case.

Reducing the timeout to 5s to make the behavior a little better.

If people live in areas with poor connection where a higher timeout
would be appropriate, I'd argue that they should increase the timeout
rather than requiring everbody else to set it to a non-zero value.

Additionally, communicate how often we're re-trying.

[1] Because of `download-attempts` from `nix.conf(5)`

Change-Id: I8500dce0c8230247dd492e43cc7af4d3b58c4710

doc/manual/rl-next/connect-timeout-default.md

@@ -0,0 +1,10 @@
+---
+synopsis: Set default of `connect-timeout` to `5`
+issues: []
+cls: [2799]
+category: Miscellany
+credits: [ma27]
+---
+
+By default, the connection timeout to substituters is now 5s instead of 300s.
+That way, unavailable substituters are detected quicker.

lix/libstore/file-transfer-settings/connect-timeout.md

@@ -2,7 +2,7 @@
 name: connect-timeout
 internalName: connectTimeout
 type: unsigned long
-default: 0
+default: 5
 ---
 The timeout (in seconds) for establishing connections in the
 binary cache substituter. It corresponds to `curl`’s

lix/libstore/filetransfer.cc

@@ -889,9 +889,9 @@ struct curlFileTransfer : public FileTransfer
             std::uniform_real_distribution<> dist(0.0, 0.5);
             int ms = parent.baseRetryTimeMs * std::pow(2.0f, attempt - 1 + dist(random));
             if (totalReceived) {
-                warn("%s; retrying from offset %d in %d ms", context, totalReceived, ms);
+                warn("%s; retrying from offset %d in %d ms (attempt %d/%d)", context, totalReceived, ms, attempt, tries);
             } else {
-                warn("%s; retrying in %d ms", context, ms);
+                warn("%s; retrying in %d ms (attempt %d/%d)", context, ms, attempt, tries);
             }
 
             std::this_thread::sleep_for(std::chrono::milliseconds(ms));