Support zero-length data items in LSM, reference #540.
Merge branch 'develop' into lsm-empty-value

Conflicts:
	src/lsm/lsm_cursor.c
Encode empty values as empty in LSM trees.

refs #540, #966, #968.
Use empty values in LSM indices.

refs #540
Merge pull request #968 from wiredtiger/lsm-empty-value

Support zero-length data items in LSM, reference #540.