sst_dump to reduce number of file reads (#6836)

Summary:
sst_dump can issue many file reads from the file system. This doesn't work well with file systems without a OS cache, especially remote file systems. In order to mitigate this problem, several improvements are done:
1. --readahead_size is added, so that users can specify readahead size when scanning the data.
2. Force a 512KB tail readahead, which prevents three I/Os for footer, meta index and property blocks and hopefully index and filter blocks too.
3. Consoldiate SSTDump's I/Os before opening the file for read. Use the same file prefetch buffer.
Pull Request resolved: facebook/rocksdb#6836

Test Plan: Add a test that covers this new feature.

Reviewed By: pdillinger

Differential Revision: D21516607

fbshipit-source-id: 3ae43526286f67b2f4a5bdedfbc92719d579b87e
Signed-off-by: Changlong Chen <levisonchen@live.cn>

file/random_access_file_reader.cc

@@ -27,6 +27,8 @@ Status RandomAccessFileReader::Read(const IOOptions& opts, uint64_t offset,
                                     AlignedBuf* aligned_buf,
                                     bool for_compaction) const {
   (void)aligned_buf;
+
+  TEST_SYNC_POINT_CALLBACK("RandomAccessFileReader::Read", nullptr);
   Status s;
   uint64_t elapsed = 0;
   {