Fix page retained size by accounting slice base object once for a page

[singcha]

Test plan -
Tested with Presto on Spark failing broadcast queries & Presto small & large queries in production

== RELEASE NOTES ==

General Changes
* Fix a memory overcounting issue for using varchar types that could cause queries to fail with exceeded memory limit errors.

[tdcmeehan]

Minor comments

[highker]

LGTM. Would be good if @rschlussel can help to take another look.

[rschlussel]

Looks good aside from existing comments.

[rschlussel]

Also, looks like some of the code was taken from #10633. you should give attribution to the author in the commit message. You can use these guidelines: https://github.com/prestodb/presto/wiki/Review-and-Commit-guidelines#attribution

[rschlussel]

You have a checkstyle violation: Error: src/test/java/com/facebook/presto/execution/buffer/TestPagesSerde.java:[138,48] (whitespace) WhitespaceAfter: ',' is not followed by whitespace.