Merge pull request docker#133 from abronan/fix_zk_wrong_list_error

zk: fix wrong error code check on List

Do not return boltdb bucket not found error

While doing a boltdb operation and if the bucket is not found
we should not return a boltdb specific bucket not found error
because this causes leaky abstraction where in the user of libkv
needs to know about boltdb and import boltdb dependencies
neither of which is desirable. Replaced all the bucket not found
errors with the more generic `store.ErrKeyNotFound` error which
is more appropriate.

Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>

Merge pull request docker#80 from abronan/cas_add_errexists

Return store.ErrKeyExists on AtomicPut