Lazy initialization for lower latency on startup

Currently, Cassandra will open each and every sstable file (I assume for pulling its metadata and create initial in-memory mapping to persisted data), when there are large amount of keyspaces and tables, this will cause a major overhead when restarting a node. 

In our use case, many keyspaces are rarely accessed and we would prefer that their initialization will be invoked lazily and not on startup, thereby reducing the time it takes a node to start serving clients.

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