Introduce a blob file reader class

[ltamasi]

Summary:
The patch adds a class called BlobFileReader that can be used to retrieve blobs
using the information available in blob references (e.g. blob file number, offset, and
size). This will come in handy when implementing blob support for Get, MultiGet,
and iterators, and also for compaction/garbage collection.

When a BlobFileReader object is created (using the factory method Create),
it first checks whether the specified file is potentially valid by comparing the file
size against the combined size of the blob file header and footer (files smaller than
the threshold are considered malformed). Then, it opens the file, and reads and verifies
the header and footer. The verification involves magic number/CRC checks
as well as checking for unexpected header/footer fields, e.g. incorrect column family ID
or TTL blob files.

Blobs can be retrieved using GetBlob. GetBlob validates the offset and compression
type passed by the caller (because of the presence of the header and footer, the
specified offset cannot be too close to the start/end of the file; also, the compression type
has to match the one in the blob file header), and retrieves and potentially verifies and
uncompresses the blob. In particular, when ReadOptions::verify_checksums is set,
BlobFileReader reads the blob record header as well (as opposed to just the blob itself)
and verifies the key/value size, the key itself, as well as the CRC of the blob record header
and the key/value pair.

In addition, the patch exposes the compression type from BlobIndex (both using an
accessor and via DebugString), and adds a blob file read latency histogram to
InternalStats that can be used with BlobFileReader.

Test Plan:
make check

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[riversand963]

Thanks @ltamasi for the PR, and I left a few minor comments.

[ltamasi]

Thanks for the review @riversand963 !

[riversand963]

LGTM except for the unresolved minors.

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[facebook-github-bot]

@ltamasi merged this pull request in 22655a3.

[mrambacher]

This change fails to build on MacOS:

db/blob/blob_file_reader_test.cc:332:29: error: constexpr variable 'expiration_range_header' must be initialized by a
constant expression
constexpr ExpirationRange expiration_range_header(1, 2);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
db/blob/blob_file_reader_test.cc:332:29: note: non-constexpr constructor 'pair<int, int, false>' cannot be used in a
constant expression
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/utility:447:5: note: declared here
pair(_U1&& __u1, _U2&& __u2)
^
db/blob/blob_file_reader_test.cc:370:29: error: constexpr variable 'expiration_range_footer' must be initialized by a
constant expression
constexpr ExpirationRange expiration_range_footer(1, 2);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
db/blob/blob_file_reader_test.cc:370:29: note: non-constexpr constructor 'pair<int, int, false>' cannot be used in a
constant expression
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/utility:447:5: note: declared here
pair(_U1&& __u1, _U2&& __u2)

[ltamasi]

Thanks for letting me know. #7519 should fix this.

[mrambacher]

This appears to have introduced a regression in the stats: #7664