Summary

WT_REF::flags should never change from WT_REF_FLAG_INTERNAL to WT_REF_FLAG_LEAF or vice versa

Motivation

Currently, in __wt_btree_new_leaf_page(), the flags in a WT_REF can change from 
WT_REF_FLAG_INTERNAL to WT_REF_FLAG_LEAF being set.
During the investigation for WT-9057, alexander.gorrod@mongodb.com and I determined that the code would be safer from the risk of race conditions if WT_REF::flags never change from WT_REF_FLAG_INTERNAL to WT_REF_FLAG_LEAF or vice versa. However, the code changes required to do this are not straightforward.

• Does this affect any team outside of WT?
  No

• How likely is it that this use case or problem will occur?
  Unlikely

• If the problem does occur, what are the consequences and how severe are they?
  It might cause a crash.

• Is this issue urgent?
  No

Acceptance Criteria (Definition of Done)
A code inspection shows that the WT_REF::flags can never change from WT_REF_FLAG_INTERNAL to WT_REF_FLAG_LEAF or vice versa, and asserts are in place to enforce the correct flag state where required.

• Testing
  Existing tests pass

• Documentation update
  N/A

There has been a lot of useful discussion in WT-9057 that is relevant for this ticket too.

 using related to, depends on, dependent on by links)