cql3: add value_for method

In order to extract value from a restriction for just one column,
value_for(column_name, options) method is implemented.
It's needed because once ALLOW FILTERING support was introduced,
index-related restrictions may contain more than 1 value.

cql3/restrictions/restrictions.hh

@@ -68,6 +68,10 @@ public:
 
     virtual std::vector<bytes_opt> values(const query_options& options) const = 0;
 
+    virtual bytes_opt value_for(const column_definition& cdef, const query_options& options) const {
+        throw exceptions::invalid_request_exception("Single value can be obtained from single-column restrictions only");
+    }
+
     /**
      * Returns <code>true</code> if one of the restrictions use the specified function.
      *

cql3/restrictions/single_column_primary_key_restrictions.hh

@@ -310,6 +310,11 @@ public:
         }
         return res;
     }
+
+    virtual bytes_opt value_for(const column_definition& cdef, const query_options& options) const override {
+        return _restrictions->value_for(cdef, options);
+    }
+
     std::vector<bytes_opt> bounds(statements::bound b, const query_options& options) const override {
         // TODO: if this proved to be required.
         fail(unimplemented::cause::LEGACY_COMPOSITE_KEYS); // not 100% correct...

cql3/restrictions/single_column_restrictions.hh

@@ -111,6 +111,11 @@ public:
         return r;
     }
 
+    virtual bytes_opt value_for(const column_definition& cdef, const query_options& options) const override {
+        auto it = _restrictions.find(std::addressof(cdef));
+        return (it != _restrictions.end()) ? it->second->value(options) : bytes_opt{};
+    }
+
     /**
      * Returns the restriction associated to the specified column.
      *

cql3/restrictions/statement_restrictions.cc

@@ -72,6 +72,9 @@ public:
         // throw? should not reach?
         return {};
     }
+    bytes_opt value_for(const column_definition& cdef, const query_options& options) const override {
+        return {};
+    }
     std::vector<T> values_as_keys(const query_options& options) const override {
         // throw? should not reach?
         return {};