Enhance cursor operations to account for prepare state

We have recently added a WT_SESSION::prepare_transaction API stub, and will implement transaction visibility on updates as part of WT-3850. Once that work is done, we need to implement code in cursor operations, so we do the right thing when an operation encounters a WT_UPDATE structure in the prepared state.

This involves changing read operations to:
Check txn ID visible.
Continue to loop if state is ‘WT_UPDATE_LOCKED’.
If state is ‘WT_UPDATE_PREPARE’
Read timestamp to local.
Is state ‘WT_UPDATE_PREPARE’.

This involves changing update operations to:
Return either WT_ROLLBACK or WT_PREPARE_CONFLICT when it encounters WT_UPDATE structures in a prepared state.