Precise index scans for NOW() and CLOCK() predicates

In 4.5, predicates involving NOW() and CLOCK() functions lead to broad scans of secondary indexes. They should instead lead to precise scans based on the actual clock values at execution time.

See https://forums.couchbase.com/t/n1ql-how-to-use-a-gsi-when-doing-a-join/8539/6