Rename index header's view_btree_states to btree_states

In order to be more generic, rename view_btree_states to view_states.
The spatial index will also use this field to store its state.

Change-Id: Ic0fd8749f270d3360aa681d75a2f102c5e5eb5d0
Reviewed-on: http://review.couchbase.org/39627
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>