propagate timeout errors properly in multiplexing cache mget

propagate timeout errors properly in multiplexing cache mget

use error from _makeNodeResolverWithTimeout

set error instead of logging

make RedisConnection return PartialResultErrors and flatten PartialResultErrors in CacheCluster

fit iteration over partialresulterror

move error flattening to PartialResultError class

fix tests

more tests

bump version

consolidate flattenErrors and flattenData in PartialResultError

fix comments