Use cached version of payment processor.

Use cached version of payment processor.

I am hitting a slightly obscure bug in Redis that is solved by this change.

Per https://github.com/eileenmcnaughton/nz.co.fuzion.omnipaymultiprocessor/issues/55
we have an issue whereby Omnipay adds the gateway to a variable on class when processing payments.

In some cases this gateway will not serialize into a key under Redis causing a fatal.

However, we don't really need it to - the payment processor that we are aiming to cache is
'as loaded' not 'as used'. The caching of the processors appears to have been removed in
the past because of issues around domains -however, I think we can get
past that by changing the cache key.