cql3: statement_restrictions: use functional style

Instead of a constructor, use a new function
analyze_statement_restrictions() as the entry point. It returns an
immutable statement_restrictions object.

This opens the door to returning a variant, with each arm of the variant
corresponding to a different query plan.

test/boost/statement_restrictions_test.cc

@@ -30,7 +30,7 @@ query::clustering_row_ranges slice(
         const std::vector<expr::expression>& where_clause, cql_test_env& env,
         const sstring& table_name = "t", const sstring& keyspace_name = "ks") {
     prepare_context ctx;
-    return restrictions::statement_restrictions(
+    return restrictions::analyze_statement_restrictions(
             env.data_dictionary(),
             env.local_db().find_schema(keyspace_name, table_name),
             statements::statement_type::SELECT,