Avoid reading lookaside pages in truncate fast path

In WT-3805, we made some changes intended to skip most lookaside data when truncating tables.

This is a particular issue for MongoDB's oplog, which can have lookaside entries when there is replication lag, but should be truncated without re-reading the oplog pages or the lookaside pages containing those entries.

It appears from internal testing that the changes in WT-3805 were not effective. That needs to be fixed, and we need a regression test for standalone WiredTiger that this isn't reintroduced.