couch_mrview: fix purge when seq_indexed=true

This change makes tsure to also purge the log btree and related btrees
in views when seqs_indexed is set to true.
couch_mrview: fix changes log

This changes make sure we pass correct keys to remove to the seq and
key_byseq btrees in views.
couch_mrview: add couch_mrview:count_view_changes_since/{4,5}

Add function to couch changes in a view index, similar to
couch_db:count_changes_since/2 .
couch_mrview: fix compaction with seqs_indexed=true

This change smakes sure to compact the view indexes when using the view
changes.