ResultSet::TaskRetries: add get_seconds_to_next_retry

Get the number of seconds before the next retriable task is ready.

This number is specifically intended to be used as a timeout, where
`undef` means never time out.

t/Schema/ResultSet/TaskRetries.t

@@ -0,0 +1,45 @@
+use strict;
+use warnings;
+use Setup;
+
+my %ctx = test_init();
+
+require Hydra::Schema;
+require Hydra::Model::DB;
+
+use Test2::V0;
+
+my $db = Hydra::Model::DB->new;
+hydra_setup($db);
+
+my $taskretries = $db->resultset('TaskRetries');
+
+subtest "get_seconds_to_next_retry" => sub {
+    subtest "Without any records in the database" => sub {
+        is($taskretries->get_seconds_to_next_retry(), undef, "Without any records our next retry moment is forever away.");
+    };
+
+    subtest "With only tasks whose retry timestamps are in the future" => sub {
+        $taskretries->create({
+            channel => "bogus",
+            pluginname => "bogus",
+            payload => "bogus",
+            attempts => 1,
+            retry_at => time() + 100,
+        });
+        is($taskretries->get_seconds_to_next_retry(), within(100, 2), "We should retry in roughly 100 seconds");
+    };
+
+    subtest "With tasks whose retry timestamp are in the past" => sub {
+        $taskretries->create({
+            channel => "bogus",
+            pluginname => "bogus",
+            payload => "bogus",
+            attempts => 1,
+            retry_at => time() - 100,
+        });
+        is($taskretries->get_seconds_to_next_retry(), 0, "We should retry immediately");
+    }
+};
+
+done_testing;