hashcash.el (hashcash-default-payment): Change default to 20

hashcash.el (hashcash-default-payment): Change default to 20
(hashcash-default-accept-payment): Change default to 20
(hashcash-process-alist): New variable
(hashcash-generate-payment-async): Add
(hashcash-already-paid-p): Add
(hashcash-insert-payment): Don't generate payments twice
(hashcash-insert-payment-async): Add
(hashcash-insert-payment-async-2): Add
(hashcash-cancel-async): Add
(hashcash-wait-async): Add
(hashcash-processes-running-p): Add
(hashcash-wait-or-cancel): Add
(mail-add-payment): New optional argument.  Conditionally start
asynchronous calculation.
(mail-add-payment-async): Add

message.el (message-send-mail): Wait for asynchronous hashcash
results.  Don't clobber existing X-Hashcash headers.
(message-setup-1): Call mail-add-payment-async when
message-generate-hashcash is non-nil.