With the changes in LUCENE-2649, the FieldCache also returns if the bit is valid or not. This is enough to support sortMissingLast=true with Trie* fields. Then we can get rid of the Sortable* fields
Add sort missing first/last ability to SortField and ValueComparator
Incorrect sort by Numeric values for documents missing the sorting field
Rework FieldCache to be more flexible/general
Trie fields should support sortMissingLast="true"