Payment processor contribution page performance improvement

Whilst investigating the development of another payment processor extension I found that 

CRM_Financial_BAO_PaymentProcessor::hasPaymentProcessorSupporting() is being called 4 times for a backoffice payment and having quite a noticeable performance impact on the "Submit Credit Card Contribution" form.

The linked PR makes the following changes:

1. Use a static variable in hasPaymentProcessorSupporting() so we only check once per request.
2. Don't load test processors when we don't need to in getPaymentProcessors()

Timings from a (slow) site:
Oct 03 17:07:06 [info] exec time old: 1.7639739513397
Oct 03 17:07:07 [info] exec time old: 1.2242529392242
Oct 03 17:07:15 [info] exec time old: 1.2430901527405
Oct 03 17:07:27 [info] exec time old: 1.2307901382446

Oct 03 17:08:27 [info] exec time new: 1.2210669517517
Oct 03 17:08:28 [info] exec time new: 0.72421288490295
Oct 03 17:08:34 [info] exec time new: 7.1525573730469E-6
Oct 03 17:08:44 [info] exec time new: 6.1988830566406E-6