Follow up from a conversation on #1521.

The FileMetadata struct now has smallest/largest pairs for points, ranges and the entire table. In the absence of a pair, comparisons elsewhere in the code use k.UserKey == nil to detect the absence of a bound, which is a little smelly. Specifically the trailer is zero, so the key could be interpreted as DEL#0.

Consider introducing a marker for "absence of bound". For example, a key with kind InternalKeyKindInvalid and a zero sequence number.