Avoid getting two snapshots when setting a read_timestamp

In WT-3981 we get a new transaction snapshot when setting a read_timestamp in a running transaction.

That means this pattern:

session->begin_transaction("isolation=snapshot")

session->timestamp_transaction("read_timestamp=X")

calls __wt_txn_get_snapshot twice.

Find a way to avoid one of the calls (while making sure that the snapshot is current as of the timestamp_transaction call for correctness). Setting a read timestamp is a relatively common case, and getting the snapshot can be expensive when many sessions are active.