multi_get/multi_set Incompatible with Python 3.6

[leorochael]

Python 3.6 has no .format() method on the bytes object ('b'strings'), but the multi_* methods try to format a prefix using it.

Additionally, on Python 3, str is unicode, but the multi_* methods are creating prefixed keys by interpolating a bytes string and, in the no-prefix case, with an empty bytes prefix, which raises an error on the multi_set() case and generates an incompatible key since b'foo' != u'foo'.