N1QL: API signature change.

- Rename Span.Equal to Span.Seek
- Index.EqualKey() to Index.SeekKey

2i still retains the local nomenclature, so the rest of the components remains
unaffected.

Change-Id: I75fdb14cb01c0c4315a2bf2018cb1d25740f1906
Reviewed-on: http://review.couchbase.org/44101
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>