Enable subcompactions in manual level-based compaction

Summary:
This is the simplest way I could think of to speed up `CompactRange`. It works but isn't that optimal because it relies on the same `max_compaction_bytes` and `max_subcompactions` options that are used in other places. If it turns out to be useful we can allow overriding these in `CompactRangeOptions` in the future.
Closes https://github.com/facebook/rocksdb/pull/3549

Differential Revision: D7117634

Pulled By: ajkr

fbshipit-source-id: d0cd03d6bd0d2fd7ea3fb13cd3b8bf7c47d11e42

HISTORY.md

@@ -6,6 +6,7 @@
 
 ### New Features
 * Avoid unnecessarily flushing in `CompactRange()` when the range specified by the user does not overlap unflushed memtables.
+* If `ColumnFamilyOptions::max_subcompactions` is set greater than one, we now parallelize large manual level-based compactions.
 * Add "rocksdb.live-sst-files-size" DB property to return total bytes of all SST files belong to the latest LSM tree.
 
 ### Bug Fixes

db/compaction.cc

@@ -446,7 +446,8 @@ bool Compaction::ShouldFormSubcompactions() const {
     return false;
   }
   if (cfd_->ioptions()->compaction_style == kCompactionStyleLevel) {
-    return start_level_ == 0 && output_level_ > 0 && !IsOutputLevelEmpty();
+    return (start_level_ == 0 || is_manual_compaction_) && output_level_ > 0 &&
+           !IsOutputLevelEmpty();
   } else if (cfd_->ioptions()->compaction_style == kCompactionStyleUniversal) {
     return number_levels_ > 1 && output_level_ > 0;
   } else {