Add support for taking weak hazard pointers on a page when adding a modification to the transaction

Transactions in WiredTiger maintain an array of modifications, adding a new one for each operation performed in the context of that transaction. In this ticket we should extend the __wt_txn_op structure to hold a hazard pointers. When the transaction performs a modification a weak hazard pointer should be taken on the page and saved in the new hazard pointer spot in the op structure.

Scope:

• Add a hazard pointer entry to __wt_txn_op
• Take weak hazard pointers where relevant, likely by passing a REF or similar to __wt_txn_modify
• Write a test for the functionality if possible