Add a WT_SESSION.app_private field for applications to use in callbacks
taking a WT_SESSION handle. Reference #1423.
Choose a more stable chunk size for small-lsm wtperf workload.

The previous size meant we sometimes didn't merge the tree down
because of the number of chunks created during the load.
Merge pull request #1459 from wiredtiger/session-app-private

Add a WT_SESSION.app_private field for applications to use in callbacks taking a WT_SESSION handle.
src/lsm/lsm_cursor.c:1203:12: warning: variable 'cmp' may be uninitialized when used here [-Wconditional-uninitialized]