[cache] Add a caching layer

[ndragazis]

The current design of the storage engine is based on Memtables and SSTables (see #4). Memtables are in-memory ordered maps implemented as std::map objects from the C++ standard library.

Opening this issue to add a caching layer on top. The cache will be an LRU write-through cache implemented over an std::unordered_map. The cache will be storing data that were recently read or written.

Since the cache trades memory with speed, the size of the cache shall be configurable by the user. The user shall also have the option to disable it.

Related content:

• https://www.scylladb.com/2018/07/26/how-scylla-data-cache-works/