[column families] Implement DB::OpenWithColumnFamilies()

Summary:
In addition to implementing OpenWithColumnFamilies, this diff also includes some minor changes:
* Changed all column family names from Slice() to std::string. The performance of column family name handling is not critical, and it's more convenient and cleaner to have names as std::strings
* Implemented ColumnFamilyOptions(const Options&) and DBOptions(const Options&)
* Added ColumnFamilyOptions to VersionSet::ColumnFamilyData. ColumnFamilyOptions are specified on OpenWithColumnFamilies() and CreateColumnFamily()

I will keep the diff in the Phabricator for a day or two and will push to the branch then. Feel free to comment even after the diff has been pushed.

Test Plan: Added a simple unit test

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15033

include/rocksdb/column_family.h

@@ -26,6 +26,6 @@ struct ColumnFamilyHandle {
 };
 
 const ColumnFamilyHandle default_column_family = ColumnFamilyHandle();
-extern const Slice& default_column_family_name;
+extern const std::string default_column_family_name;
 
 }

include/rocksdb/db.h

@@ -13,6 +13,7 @@
 #include <stdio.h>
 #include <memory>
 #include <vector>
+#include <string>
 #include "rocksdb/iterator.h"
 #include "rocksdb/options.h"
 #include "rocksdb/types.h"
@@ -26,8 +27,11 @@ struct ColumnFamilyHandle;
 extern const ColumnFamilyHandle default_column_family;
 
 struct ColumnFamilyDescriptor {
-  Slice name;
+  std::string name;
   ColumnFamilyOptions options;
+  ColumnFamilyDescriptor(const std::string& name,
+                         const ColumnFamilyOptions& options)
+      : name(name), options(options) {}
 };
 
 // Update Makefile if you change these
@@ -117,7 +121,7 @@ class DB {
   // Create a column_family and return the handle of column family
   // through the argument handle.
   virtual Status CreateColumnFamily(const ColumnFamilyOptions& options,
-                                    const Slice& column_family,
+                                    const std::string& column_family_name,
                                     ColumnFamilyHandle* handle);
 
   // Drop a column family specified by column_family handle.

include/rocksdb/options.h

@@ -68,6 +68,8 @@ struct CompressionOptions {
                                                        strategy(strategy){}
 };
 
+struct Options;
+
 struct ColumnFamilyOptions {
   // -------------------
   // Parameters that affect behavior
@@ -426,6 +428,8 @@ struct ColumnFamilyOptions {
 
   // Create ColumnFamilyOptions with default values for all fields
   ColumnFamilyOptions();
+  // Create ColumnFamilyOptions from Options
+  explicit ColumnFamilyOptions(const Options& options);
 };
 
 struct DBOptions {
@@ -627,6 +631,8 @@ struct DBOptions {
 
   // Create DBOptions with default values for all fields
   DBOptions();
+  // Create DBOptions from Options
+  explicit DBOptions(const Options& options);
 };
 
 // Options to control the behavior of a database (passed to DB::Open)