feat(worker): scale connection pool with worker threads

This will automatically increase the DB connection pool size if the number of threads needed in a worker is less than the maximum pool size configured.

spec/lib/postal_spec.rb

@@ -9,4 +9,10 @@ RSpec.describe Postal do
       expect(Postal.signer.private_key.to_pem).to eq OpenSSL::PKey::RSA.new(File.read(Postal::Config.postal.signing_key_path)).to_pem
     end
   end
+
+  describe "#change_database_connection_pool_size" do
+    it "changes the connection pool size" do
+      expect { Postal.change_database_connection_pool_size(8) }.to change { ActiveRecord::Base.connection_pool.size }.from(5).to(8)
+    end
+  end
 end