[docs] Add basic advice on setting block cache size
[docs] Add basic advice on setting block cache size

This adds a short section to the troubleshooting guide about improving
the performance of the block cache. It's fuzzy since the
effectiveness of the cache and the efficacy of enlarging it are so
workload dependent (e.g. consider a workload doing full table scans vs.
one mostly re-scanning a small range checking for updates), but I tried
to provide a starting point for users to evaluate their cache size since
we've totally lacked any advice on that up to this point.

I also added information about the change due to release in 1.8 that
servers won't start when the block cache capacity is set too large
relative to the memory limit.

Change-Id: Idc7411c38b6fcc8694509ec89c32e2fe74e6c0db
Reviewed-on: http://gerrit.cloudera.org:8080/11420
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Tested-by: Kudu Jenkins
Reviewed-by: Andrew Wong <b68e4fdc6430321a6b47400732ff97d7ae91234e@cloudera.com>

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