Handle full final subcompaction output file with range deletions
Handle full final subcompaction output file with range deletions

Summary:
This conditional should only open a new file that's dedicated to range deletions when it's the sole output of the subcompaction. Previously, we created such a file whenever the table builder was nullptr, which would've also been the case whenever the CompactionIterator's final key coincided with the final output table becoming full.
Closes https://github.com/facebook/rocksdb/pull/1507

Differential Revision: D4174613

Pulled By: ajkr

fbshipit-source-id: 9ffacea

wat-ze-hex/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb