Change Shard to use safe_find_or_create_by

Using this method allows us to benefit from a recent optimization where
the transaction is skipped in the most common scenario where the record
is found on the first attempt

app/models/shard.rb

@@ -18,10 +18,6 @@ class Shard < ApplicationRecord
   end
 
   def self.by_name(name)
-    transaction(requires_new: true) do
-      find_or_create_by(name: name)
-    end
-  rescue ActiveRecord::RecordNotUnique
-    retry
+    safe_find_or_create_by(name: name)
   end
 end

spec/models/shard_spec.rb

@@ -33,19 +33,21 @@ RSpec.describe Shard do
       expect(result.name).to eq('foo')
     end
 
-    it 'retries if creation races' do
+    it 'returns existing record if creation races' do
+      shard_created_by_others = double(described_class)
+
       expect(described_class)
-        .to receive(:find_or_create_by)
-        .with(name: 'default')
-        .and_raise(ActiveRecord::RecordNotUnique, 'fail')
-        .once
+        .to receive(:find_by)
+        .with(name: 'new_shard')
+        .and_return(nil, shard_created_by_others)
 
       expect(described_class)
-        .to receive(:find_or_create_by)
-        .with(name: 'default')
-        .and_call_original
+        .to receive(:create)
+        .with(name: 'new_shard')
+        .and_raise(ActiveRecord::RecordNotUnique, 'fail')
+        .once
 
-      expect(described_class.by_name('default')).to eq(default_shard)
+      expect(described_class.by_name('new_shard')).to eq(shard_created_by_others)
     end
   end
 end