Allow a configurable number of background threads.

Summary:
The background threads are necessary for compaction.
For slower storage, it might be necessary to have more than
one compaction thread per DB. This patch allows creating
a configurable number of worker threads.
The default reamins at 1 (to maintain backward compatibility).

Test Plan:
run all unit tests. changes to db-bench coming in
a separate patch.

Reviewers: heyongqiang

Reviewed By: heyongqiang

CC: MarkCallaghan

Differential Revision: https://reviews.facebook.net/D5559

include/leveldb/env.h

@@ -155,6 +155,10 @@ class Env {
   virtual Status GetAbsolutePath(const std::string& db_path,
       std::string* output_path) = 0;
 
+  // The number of background worker threads for this environment.
+  // default: 1
+  virtual void SetBackgroundThreads(int number) = 0;
+
  private:
   // No copying allowed
   Env(const Env&);
@@ -344,6 +348,9 @@ class EnvWrapper : public Env {
       std::string* output_path) {
     return target_->GetAbsolutePath(db_path, output_path);
   }
+  void SetBackgroundThreads(int num) {
+    return target_->SetBackgroundThreads(num);
+  }
 
  private:
   Env* target_;