Separate LAST cache from MTree

Currently, we have a cache in MTree to store the LAST value of timeseries, in the distributed version, however, the metadata and data are stored separately, and when the metadata is needed, it will be pulled from another node and stored in a local cache, resulting in that such timeseries does not exist in MTree and its LAST cache is accessed, a PathNotExistException will be raised.

So we temporarily disable the LAST cache of the remotely existing timeseries, but we hope we can modify it so that we can still cache the last value of those timeseries.

Here is one possible solution: abstract a LastCacheManager to manage all last caches instead of storing them in MTree, so even when the timeseries does not exist locally, its last value can still be cached and visited.