util-core: optimize ConcatBuf.isEmpty
util-core: optimize ConcatBuf.isEmpty

Problem / Solution

Rather than walking ConcatBuf's full `chain` we can determine non-emptiness
eagerly.

RB_ID=897873

BuoyantIO/twitter-util,BuoyantIO/twitter-util,BuoyantIO/twitter-util,twitter/util,twitter/util,twitter/util,BuoyantIO/twitter-util