Allow LSM trees to discard the btree handle from the active chunk.
Clean up layering violation in LSM code.
It is not safe to set the DISCARD_CLOSE flag before a checkpoint.

This change alters the code to allow WT_BTREE_DISCARD_CLOSE to be passed
via a checkpoint. Also fixes a bug where we weren't unlocking a closed
btree handle properly.
Merge branch 'develop' into lsm-discard-btree
Don't clear WT_BTREE_DISCARD_CLOSE in session_get_btree
Ensure WT_BTREE_DISCARD_CLOSE is cleared when a handle is closed.

Not strictly required for the current LSM case since we never open
the handle again.
src/session/session_btree.c: function with return after a jump to an error label
	function @ line: 99
Merge pull request #441 from wiredtiger/lsm-discard-btree

Allow LSM trees to discard the btree handle from the active chunk.