[Refactor]: Do early return in 'ProgressTracker::shouldContinueVisiting()'
[Refactor]: Do early return in 'ProgressTracker::shouldContinueVisiting()'

It is a good practice to return as early as possible from a function than
carrying around the return status in a variable and then doing a return
finally. Also this is inline with our coding guidelines.

Change-Id: I22fea54285518afb97ce991f864732f7952244e6
Reviewed-on: http://review.couchbase.org/83928
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine