[CF] Rethinking ColumnFamilyHandle and fix to dropping column families

Summary:
The change to the public behavior:
* When opening a DB or creating new column family client gets a ColumnFamilyHandle.
* As long as column family handle is alive, client can do whatever he wants with it, even drop it
* Dropped column family can still be read from (using the column family handle)
* Added a new call CloseColumnFamily(). Client has to close all column families that he has opened before deleting the DB
* As soon as column family is closed, any calls to DB using that column family handle will fail (also any outstanding calls)

Internally:
* Ref-counting ColumnFamilyData
* New thread-safety for ColumnFamilySet
* Dropped column families are now completely dropped and their memory cleaned-up

Test Plan: added some tests to column_family_test

Reviewers: dhruba, haobo, kailiu, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16101

db/compaction.cc

@@ -43,6 +43,7 @@ Compaction::Compaction(Version* input_version, int level, int out_level,
       is_full_compaction_(false),
       level_ptrs_(std::vector<size_t>(number_levels_)) {
 
+  cfd_->Ref();
   input_version_->Ref();
   edit_ = new VersionEdit();
   edit_->SetColumnFamily(cfd_->GetID());
@@ -56,6 +57,11 @@ Compaction::~Compaction() {
   if (input_version_ != nullptr) {
     input_version_->Unref();
   }
+  if (cfd_ != nullptr) {
+    if (cfd_->Unref()) {
+      delete cfd_;
+    }
+  }
 }
 
 bool Compaction::IsTrivialMove() const {
@@ -168,6 +174,12 @@ void Compaction::ReleaseInputs() {
     input_version_->Unref();
     input_version_ = nullptr;
   }
+  if (cfd_ != nullptr) {
+    if (cfd_->Unref()) {
+      delete cfd_;
+    }
+    cfd_ = nullptr;
+  }
 }
 
 void Compaction::ReleaseCompactionFiles(Status status) {