Set session btree to HS when operating on history store

format-stress-ppc-zseries-test failed on ~ Ubuntu 18.04 PPC

Host: ubuntu1804-ppc-13.pic.build.10gen.cc
Project: WiredTiger (develop)
Commit: diff: WT-6811 Allow older readers to read behind a mixed-mode operation. (#6091)

• Reinsert values behind a mixed mode operation into the history store with 0 start and stop timestamps.
• Stop converting WT_TS_NONE to WT_TS_MAX in the history store search code, instead fix it up before passing it in.
• Delete all places that we clear the history store after inserting updates as we cant guarantee ordering after inserting an update.
• Remove now unused stat.
• Rename update obsolete flag.
• Ignore tombstones on the zero timestamp search cursor as that may prevent it from placing itself correctly.
• Adding unit tests for various older reader scenarios.

Co-authored-by: Alex Cameron <alex.cameron@10gen.com> | 28 Oct 20 00:23 UTC
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