WT_WITH_SCHEMA_LOCK, WT_SESSION_SCHEMA_LOCKED don't need to spin

@agorrod, would you please review?

Reference #1202.
There is a subtle difference here. The old code used to set and clear WT_SESSION_SCHEMA_LOCKED if skip_schema_lock was set. I don't know if that's strictly required, but I suspect that we at least trigger some asserts if that changes.