Migrate checks handlesType too frequently

migrate_field_handler_invoke_all is called twice per field attached to a node. Within that function, all field types are checked for for whether they are handled or not. This happens for every class (C) for every field (F) of every entity being imported (E), leading to c*f*e number of iterations. on 1000 entities with 11 fields, this results in 242,000 calls to the handlesType() function.

It appears that a simple static cache can reduce this pretty substantially. I opted to add an iteration loop to warm that cache, and call the handlesType() function. caching the results by type and classname. The results of this reduced the number of calls to handlesType to 55. This results in a performance savings of about 0.5 seconds per 1000 items. For the migration I'm running, that's about 3hrs of savings.