Bump the default hfile.block.cache.size because of HFileV2

Here's the email I sent to the mailing list describing the problem:

A thought just stuck me while I was writing down a more detailed block
caching documentation: with HFileV2, the indexes now live in the block
cache which means that those who upgrade may all of a sudden get
terrible cache hit ratios because of all that memory taken by the
indexes. This is somewhat mitigated by the fact that people don't
usually need to keep all the index blocks in memory so in the end
we're more efficient.

Which brings me to a question: should we set hfile.block.cache.size
higher since indexes are now kept in the block cache? Currently it's
set to 20%.

Looking over my own production machines I see that the
storefileIndexSize is around 600-700MB so that's potentially how much
more data I'd have to block cache (more likely it's half of that
that's really being used actively).

What would be a good new default? 25%? 30%? How do we handle those
that will be pushed over the BC+memstore size limit because of that
change?

I'll bump this to 25% and put in the release note the fact that people should verify their settings before upgrading to make sure memstore+block cache isn't over 80% (meaning they'd haven't change the block cache size but would have bumped the memstores from 40% to 60%).