various bugfixes and improvements for view compaction

View compaction was broken after the switch to signature-based index
files, but we don't have tests for it yet so we didn't notice.  I also
committed a few changes that should make compaction faster and more
robust:

* commit the header immediately after compaction finishes.  We used
  to wait 1 seconds, but if the server restarted in that second the
  index would be reset.

* unlink from old index file at the end. Prevents process crashes
  that could couch_view (and with the delayed commit would sometimes
  cause index resets).
  
* don't wait for running view updates to finish before replacing old
  view index file.  If an update is running, restart it and point it to
  the new view group.  This alleviates the situation where the view
  compaction goes into a busy wait, printing "still behind main file"
  1000s of times to the log, and generally makes compaction finish more
  quickly.
  
* better logging


git-svn-id: https://svn.apache.org/repos/asf/couchdb/trunk@796805 13f79535-47bb-0310-9956-ffa450edef68

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