Minor improve HDFS ReadBuffer for read end of file

[KevinyhZou]

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

While read small files from hdfs by gluten, we found that it will cost more times when compare to directly query by spark.

In my testcases, the table contains 3 rows data, when query select * from $table by use HDFSReadBuffer, it will cost 1.5 seconds to read, as by spark this cost is 0.2 second.

And we found the time is cost mostly while read the file end: it will check the read cursor whether beyond the file length in libhdfs3, and get file length will take the mostly cost. So we make a improvment：get and keep the file length in a variable at the construction function, and so avoid to get file length to compare every time.

And by use this, the time cost is equals to spark in such query above.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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[robot-clickhouse]

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