assuan: Use exponential decay for first 1s of spinlock.

* common/asshelp.c (wait_for_sock): instead of checking the socket
every second, we check 10 times in the first second (with exponential
decay).

This cuts the wall clock time for the standard test suite roughly by
half.

• GnuPG-bug-id: T3490
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>