Move asynchronous calls in debbugs to SOAP function level.
* packages/debbugs/debbugs-gnu.el (top): Don't require `async'.
(debbugs-gnu-default-hits-per-page): Remove.
(debbugs-gnu-show-reports): Do not call `debbugs-get-status'
asynchronously anymore.
* packages/debbugs/debbugs.el (soap-invoke-async, async-start)
(async-get): Declare.
(debbugs-max-hits-per-request): New defconst.
(debbugs-soap-invoke-async-object): New defvar.
(debbugs-soap-invoke-async): New defun.
(debbugs-get-status): Use them.