cql3: grammar: make 'relation' return an expression rather than append to a vector
cql3: grammar: make 'relation' return an expression rather than append to a vector

The 'relation' production is self-contained, except for its interface
to the rest of the grammar where it appends to a vector of expressions
(that happens to represent a conjunction of relations). Make it stand-
alone by returning an expression, and move the responsibility for
appending to an expression vector to the whereClause production (later
we can make it build a conjunction expression rather than a vector
of expressions, paving the way for more boolean operators).

Closes #10623

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla