Resolve how to handle missed intervals

[awestendorf]

There's a disconnect in how missed intervals are handled in the reading functions. For example, MongoBackend._series does this:

for record in cursor:
  while buckets and buckets[0] < record['interval']:
    rval[ config['i_calc'].from_bucket(buckets.pop(0)) ] = self._type_no_value()

The problem here is that _type_no_value assumes the default type for that timeseries, but when using custom read functions, these filler values don't necessarily match the type that the user-provided functions are assuming.

This would also clear up issues with custom read functions, such as in this case where interval is necessary in the return data, but not strictly necessary to satisfy the goals of the library.