Support local cache

[wangxiaoying]

Adding a local cache to speed up loading frequently accessed data.

A naive solution is to maintain a LRU cache of query results (e.g., dump to parquet files). When a new query is issued, first check whether a cache of the same query exists (has equivalent query plan), load and return the cache directly if exists otherwise issue the query to DB.

Here are some questions that we can discuss:

• How to synchronize the cache data with same data in DB or should we assume source data is static?
• How to increase the opportunity of reuse (e.g., load and filter on cache if a query's result is a subset of a cache)?
• Should we use a better cost model instead of using simple LRU?
• Should we consider multiple layout selection of the cache?
• The context to use for the cache: user credential, query string, +....?
• Aging/safe disposal of the cache.