add couch_mrview:view_changes_since/{6,7} function

This function add the possibility to get changes in a view since the
last upddated sequence. You can all changes in a view since a sequence
or all changes for a key or a range in a view.

The following new secondaries are created to allows this feature:

- a generic log index to log the latest changes in views for a docid :
{DocId, [{ViewId, {Key, Seq, OP}}]} where OP can be del or add. This
index allows us to mark a key as removed if needed. It will be useful
later to help us to chain map/reduces operations or such things.

- a seq index associated to a view id : {ViewId, [{Seq, Key}, {DocId, Val}]} to look for all changes in a view

- an index indexing keys by seq: {ViewId, [{[Key, Seq], DocId}, Val}]},
  to looks for changes associated to a key or a ranhge

Note: all deleted keys are marked as deleted in the log index and their
value is {[{<<"_removed">>, true}]}.

To start to index changes you need to pass the options {seq_indexed:
true} to the design document.

Caveat: when the changes are indexed the size of the index is significantly higher.

Example of usage:  https://www.friendpaste.com/5Y6gihQReaxd8ERqbDom3y