Use `Cache-Control: max-age=0' instead of `no-cache'.

Varnish ignores `no-cache', and since `max-age=0' works just as well
for both Varnish and web browsers, and makes for simpler code, just
use that instead.
Add protobuf as a dependency in preparation for AsyncHBase 1.5.
Stagger the compaction of rows.

Instead of always flushing all the rows in order, which causes hot spots
in HBase during heavy flushes, we now randomly skip some rows, so as to
better stagger the compactions across multiple regions.
Update AsyncHBase to 1.5.0
Merge remote-tracking branch 'remotes/official/next'
Merge branch 'master' into next