Unfix view compaction progress reports

This reverts the compaction task status progress back to what exists on
1.2.x. The issue is that we use the number of documents in the database
instead of the number of docids in the union of all views (the number of
rows in the view's id btree). In this particular case the desire to have
seamless view upgrades outweighs fixing the relatively minor UI bug.

backport from couchdb 1.3.x branch