Increase of compaction threads should be logged at info level instead of a warning

Summary:
This log message shouldn't be a warning; some services are seeing high warning count due to this.

The count for the below line is a few hundreds of millions, as per Logview:
```
[rocksdb/src/db/column_family.cc:729] [checkpoints] Increasing compaction threads because we have 2 level-0 files
```
Closes https://github.com/facebook/rocksdb/pull/2364

Differential Revision: D5123565

Pulled By: sagar0

fbshipit-source-id: a07ce499a4f82f0ebde9cda9f4948fb9df6a734c

db/column_family.cc

@@ -729,7 +729,7 @@ void ColumnFamilyData::RecalculateWriteStallConditions(
               mutable_cf_options.level0_slowdown_writes_trigger)) {
         write_controller_token_ =
             write_controller->GetCompactionPressureToken();
-        ROCKS_LOG_WARN(
+        ROCKS_LOG_INFO(
             ioptions_.info_log,
             "[%s] Increasing compaction threads because we have %d level-0 "
             "files ",
@@ -743,7 +743,7 @@ void ColumnFamilyData::RecalculateWriteStallConditions(
         write_controller_token_ =
             write_controller->GetCompactionPressureToken();
         if (mutable_cf_options.soft_pending_compaction_bytes_limit > 0) {
-          ROCKS_LOG_WARN(
+          ROCKS_LOG_INFO(
               ioptions_.info_log,
               "[%s] Increasing compaction threads because of estimated pending "
               "compaction "