Fix a deadlock in LSM

Move the LSM tree rwlock inside the schema lock to avoid a deadlock (see #749).

The big issue here is that we need to grab the schema lock before calling `__clsm_open_cursors`, because that function reads the LSM tree structure (so it needs a read lock), but it will probably also need the schema lock because it is opening some chunks for the first time in this session.  It is unclear what impact this will have on performance.