Track data handles rather than btree handles.  This shouldn't be a real change, but tightens up some assertions.
Wait for the handle list lock when discarding in-memory chunks from an LSM tree.  This avoids a trylock that has to fight with eviction walks, so we will discard chunks more consistently.

refs #1391
Fix a race between creating and opening an LSM tree that gave an off-by-one reference count.