The intention of sql.RawBytes is for it to hold memory owned by the
database. When used, it's content is only valid until the `Next`,
`Scan` or `Close` is called on the `Rows`
To ensure that we meet this behaviour, when `[]byte` is used in a
column, it's value is copied to a buffer that we keep track of for
later invalidation. By doing this, incorrect use of `sql.RawBytes`
values is exposed in tests that use go-sqlmock. Without this, when a
real database is used and it's driver does share memory, then those
issues would not be exposed until runtime (and in non-obvious ways)
