DHT: Remove per-process ChunkCache

Performance testing has indicated the per-process ChunkCache isn't
very effective for the DHT storage implementation.  If a server is
using the DHT storage backend, it is most likely part of a larger
cluster where requests are distributed in a round-robin fashion
between the member servers.

In such a scenario there is insufficient data locality between
requests to get a good hit ratio on the per-process ChunkCache.  A low
hit ratio means the cache is actually hurting performance by eating up
memory that could otherwise be used for transient request data, and
increasing pressure on the GC when it needs to find free space.

Remove all of the ChunkCache code.  Installations that want to cache
(to reduce database usage) should wrap their Database with a
CacheDatabase and use a network based CacheServer.

I left the ChunkCache in the original DHT storage commit because I
wanted to document in the history of the project that its probably
worth *not* having, but leave open a door for someone to revert this
change if they find otherwise at a later date.

Change-Id: I364d0725c46c5a19f7443642a40c89ba4d3fdd29
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

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