Partner ticket to WT-11285 which adds WT_READ_ONCE.

 
In WiredTiger the WT_PUBLISH macro is currently used for two purposes:

1. Providing volatile write semantics which prevents fused writes (taking two write to memory and fusing them into a single write under the hood) and introduced write (taking a single write to memory and converting it into multiple writes under the hood)
2. Providing write ordering between two variables. Ensuring that if the code says to write variable A before B that neither the compiler nor CPU execution is allowed to optimise the code to instead write B first.

PM-3221 is splitting these two purposes out into separate macros for clearer readability, and this ticket introduces the WT_WRITE_ONCE macro to handle case 1. Once all uses of WT_PUBLISH to enforce volatile semantics have been updated to use WT_WRITE_ONCE, WT_PUBLISH will be used exclusively to handle case 2.