Change thrift source folder

Set Scrooge settings so they point to the IDL
files in zipkin-thrift. Then we can compile IDLs without
relying on symlinks (which doesn't play well cross-platform)

Signed-off-by: Jeff Smick <jeff@twitter.com>

RB_ID=376821
Improve WriteQueueWorkers' pushback

For storage engines that do most of their work under the returned future (such as hbase),
having the WriteQueueWorker return immediately means that the write queue is not an
accurate representation of the amount of outstanding work.

Signed-off-by: Jeff Smick <jeff@twitter.com>

RB_ID=376841
sync oss