Skip the fast-append key comparison unless we've previously appended
data to the tree.
Comment cleanup.
Don't do an append test regardless if searching the tree, but more
importantly, don't clear the "appending" flag for a read operation.
This makes no difference in terms of performance, but means we don't
let read operations interfere with our understanding of an appending
workload.
Merge pull request #1003 from wiredtiger/row-skip-append-check

Row skip append check
Change search-near to always set the "insert" flag, LSM depends on it.
Merge branch 'row-skip-append-check' into develop
Use the default cache size in test/thread: 5MB causes odd side effects that make the test slow, particularly with valgrind.
Fix a leak of the buffer used to hold config files.
Speed up checkpoints by doing a better job of skipping pages that can't contain changes that need to be included.

Before the checkpoint pass, we have written every dirty leaf page at read-committed isolation.  So the only pages that checkpoint has to write are internal pages, or leaf pages that were dirtied after the "write leaves" phase but before the checkpoint transaction started.  Now that we separate out the allocation of a transaction ID from setting up a snapshot, we can distinguish between update transactions and the transaction we're using for the checkpoint.

refs #954, #963, #1001