Add `#has_family?`, `#has_column?`, `#delete_family`, `#delete_column`

The first two are predicates to determine if a given operation (get,
put, delete or scan) mentiones the given family or column (as
described by qualifier name and decoded qualifiers.)

Beware that `#has_column?` on mutations implies a linear scan of the
contained KeyValues.

`#delete_family` and `#delete_column` are similar to `#add_family` and
`#add_column`, but they only operate on deletes and can optionally
support a timestamp.

These methods are added to both `CellType` and `Relation` (as
delegates.)