e-sink.el: save-excursion before moving point around

For the e-sink buffer to be usable by the user, make sure that we
don't never move the point.  This simply requires that we wrap all the
point-moving code in a save-excursion.
e-sink.el: fix startup time

The first two options that run-at-time() take are: when to execute
first, and how often to repeat.  Give the startup a different (and
much smaller value) than the repeat time.  While at it, use the the
repeat parameter of run-at-time(), instead of setting up a repeat by
hand in e-sink-insert-from-temp().