Added key column type

[kelindar]

This PR adds a new Key column type which is essentially a string column with a map[string]uint32 currently. This allows for O(1) lookups by key in case where the row offset is not known. This adds an extra map lookup of course.

• Added UpdateAtKey() method to both transaction and collection. This allows to update or insert (upsert) an element at a specified key.
• Added SelectAtKey() method to both transaction and collection. This allows to read a row by its key.
• Renamed Insert() to InsertObject() and changed the signature of the existing insert methods.
• Added Insert() and InsertWithTTL() methods that take func(column.Cursor) error instead of a map[string]interface{} to keep the API consistent and not require a map when not needed.
• Fixed some buffer types