Move the WT_CBT_CONFLICT_CHECK flag from the btree-specific cursor
structure into the WT_CURSOR structure as WT_CURSTD_CONFLICT_CHK.

It's possible we could return from the row-search function with a match
not found in the insert list, check for a NULL insert before indirecting
through it.
Trivial style changes: join a pair of lines, re-order a test so it looks
more like the test that precedes it.
Don't use a cursor flag for update checks: override the insert method instead.  This gets conflict checking out of the common fast path for updates, and avoids a new flag in the public API that we don't want to document.

refs #1047, #1057, #1060
Wrap update_check in a public cursor call, so that session->dhandle is set correctly.

refs #1060
Merge pull request #1060 from wiredtiger/lsm-snapshot-update-check-only-2

Lsm snapshot update check only, review changes.