FIFO compaction style

Summary:
Introducing new compaction style -- FIFO.

FIFO compaction style has write amplification of 1 (+1 for WAL) and it deletes the oldest files when the total DB size exceeds pre-configured values.

FIFO compaction style is suited for storing high-frequency event logs.

Test Plan: Added a unit test

Reviewers: dhruba, haobo, sdong

Reviewed By: dhruba

Subscribers: alberts, leveldb

Differential Revision: https://reviews.facebook.net/D18765

include/rocksdb/options.h

@@ -53,8 +53,18 @@ enum CompressionType : char {
 };
 
 enum CompactionStyle : char {
-  kCompactionStyleLevel = 0x0,     // level based compaction style
-  kCompactionStyleUniversal = 0x1  // Universal compaction style
+  kCompactionStyleLevel = 0x0,      // level based compaction style
+  kCompactionStyleUniversal = 0x1,  // Universal compaction style
+  kCompactionStyleFIFO = 0x2,       // FIFO compaction style
+};
+
+struct CompactionOptionsFIFO {
+  // once the total sum of table files reaches this, we will delete the oldest
+  // table file
+  // Default: 1GB
+  uint64_t max_table_files_size;
+
+  CompactionOptionsFIFO() : max_table_files_size(1 * 1024 * 1024 * 1024) {}
 };
 
 // Compression options for different compression algorithms like Zlib
@@ -429,6 +439,9 @@ struct ColumnFamilyOptions {
   // The options needed to support Universal Style compactions
   CompactionOptionsUniversal compaction_options_universal;
 
+  // The options for FIFO compaction style
+  CompactionOptionsFIFO compaction_options_fifo;
+
   // Use KeyMayExist API to filter deletes when this is true.
   // If KeyMayExist returns false, i.e. the key definitely does not exist, then
   // the delete is a noop. KeyMayExist only incurs in-memory look up.