[RocksDB] cleanup EnvOptions

Summary:
This diff simplifies EnvOptions by treating it as POD, similar to Options.
- virtual functions are removed and member fields are accessed directly.
- StorageOptions is removed.
- Options.allow_readahead and Options.allow_readahead_compactions are deprecated.
- Unused global variables are removed: useOsBuffer, useFsReadAhead, useMmapRead, useMmapWrite

Test Plan: make check; db_stress

Reviewers: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11175

table/table_test.cc

@@ -293,7 +293,7 @@ class TableConstructor: public Constructor {
   TableConstructor();
 
   static uint64_t cur_uniq_id_;
-  const StorageOptions soptions;
+  const EnvOptions soptions;
 };
 uint64_t TableConstructor::cur_uniq_id_ = 1;