Cache Paypal plans.

* src/paypal.c (plan_cache, plan_counter): New.
(cache_plan_id): new.
(find_plan): Use the cache.
(paypal_find_create_plan): Insert a new plan in the cache.

Paypal has no API to lookup the plan by name. Thus we need to iterate
over all plans to find a matching one. This new cache thus avoids
long Paypal transactions.

• Signed-off-by: Werner Koch <wk@gnupg.org>