On WT_PREPARE_CONFLICT, read operations in wiredtiger_record_store.cpp and wiredtiger_index.cpp call waitUntilPreparedUnitOfWorkCommitsOrAborts and retry, roughly where we now do WT_READ_CHECK.
Also add failpoint to make WT_READ_CHECK occasionally return WT_PREPARE_CONFLICT.
Add new testing enabling this failpoint to check proper handling.