Prettified and documented io_zclient
Sketched var_handle and var_store_backend.
Facilitate todo_item_t retry.
Refactored var_base to use var_store.
Rearranged io_zclient a bit.
Started work on io_var_store.
Refactored bq_zookeeper, io_zclient and io_zhandle.

Make them saner to use with the pseudo-synchronous Zookeeper API.
Merge remote-tracking branch 'remotes/github/zrefactoring' into zrefactoring

Conflicts:
	Makefile
	pd/zk_vars/var_base.C