new protocol parameter for storage cache

Built upon !12 (merged), this MR adds a new protocol parameter cache_capacity indicating the upper bound of cache size.

Considering the capacity can be up to mega-bytes, the original design that kept cache size computed as in int,namely UInt16, will be insufficient, even if the size of value is computed as kilo-Bytes.

The test_cache is now outdated comparing to current design and thus requires rewriting entirely. Therefore, it's temporarily removed from testing suit.