Change civicrm_payment_processor.payment_processor_type to an id

Modify xml/schema/Financial/PaymentProcessor.xml as follows:
Drop
<field>
<name>payment_processor_type</name>
<type>varchar</type>
<length>255</length>
<comment>Payment Processor Type.</comment>
<add>1.8</add>
</field>
Add payment_processor_type_id as an fk to that table, with index.

Create an upgrade script snippet that creates payment_processor_type_id field, updates the field correctly based on values in payment_processor_type field, then drops payment_processor_type field, and put in the appropriate file.