Enable IO Uring in MultiGet in direct IO mode (#6815)
Summary: Currently, in direct IO mode, `MultiGet` retrieves the data blocks one by one instead of in parallel, see `BlockBasedTable::RetrieveMultipleBlocks`. Since direct IO is supported in `RandomAccessFileReader::MultiRead` in https://github.com/facebook/rocksdb/pull/6446, this PR applies `MultiRead` to `MultiGet` so that the data blocks can be retrieved in parallel. Also, in direct IO mode and when data blocks are compressed and need to uncompressed, this PR only allocates one continuous aligned buffer to hold the data blocks, and then directly uncompress the blocks to insert into block cache, there is no longer intermediate copies to scratch buffers. Pull Request resolved: https://github.com/facebook/rocksdb/pull/6815 Test Plan: 1. added a new unit test `BlockBasedTableReaderTest::MultiGet`. 2. existing unit tests and stress tests contain tests against `MultiGet` in direct IO mode. Reviewed By: anand1976 Differential Revision: D21426347 Pulled By: cheng-chang fbshipit-source-id: b8446ae0e74152444ef9111e97f8e402ac31b24f
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