ARROW-3851: [C++] Run clang-format in parallel
ARROW-3851: [C++] Run clang-format in parallel

Author: Antoine Pitrou <antoine@python.org>

Closes #3011 from pitrou/ARROW-3851-clang-format-slow and squashes the following commits:

aa65619b <Antoine Pitrou> ARROW-3851:  Run clang-format in parallel

renesugar/arrow,majetideepak/arrow,xhochy/arrow,majetideepak/arrow,apache/arrow,renesugar/arrow,apache/arrow,icexelloss/arrow,tebeka/arrow,wesm/arrow,pcmoritz/arrow,majetideepak/arrow,itaiin/arrow,wesm/arrow,majetideepak/arrow,cpcloud/arrow,majetideepak/arrow,laurentgo/arrow,laurentgo/arrow,apache/arrow,pcmoritz/arrow,majetideepak/arrow,xhochy/arrow,apache/arrow,wesm/arrow,xhochy/arrow,cpcloud/arrow,wesm/arrow,majetideepak/arrow,laurentgo/arrow,pcmoritz/arrow,kou/arrow,renesugar/arrow,renesugar/arrow,apache/arrow,itaiin/arrow,wesm/arrow,renesugar/arrow,itaiin/arrow,cpcloud/arrow,apache/arrow,icexelloss/arrow,tebeka/arrow,wesm/arrow,icexelloss/arrow,itaiin/arrow,renesugar/arrow,laurentgo/arrow,icexelloss/arrow,pcmoritz/arrow,wesm/arrow,apache/arrow,laurentgo/arrow,apache/arrow,xhochy/arrow,icexelloss/arrow,tebeka/arrow,laurentgo/arrow,xhochy/arrow,apache/arrow,pcmoritz/arrow,xhochy/arrow,kou/arrow,cpcloud/arrow,icexelloss/arrow,cpcloud/arrow,cpcloud/arrow,kou/arrow,kou/arrow,cpcloud/arrow,laurentgo/arrow,pcmoritz/arrow,wesm/arrow,itaiin/arrow,renesugar/arrow,itaiin/arrow,tebeka/arrow,kou/arrow,kou/arrow,majetideepak/arrow,renesugar/arrow,laurentgo/arrow,laurentgo/arrow,majetideepak/arrow,wesm/arrow,cpcloud/arrow,pcmoritz/arrow,cpcloud/arrow,xhochy/arrow,pcmoritz/arrow,pcmoritz/arrow,kou/arrow,cpcloud/arrow,majetideepak/arrow,cpcloud/arrow,xhochy/arrow,laurentgo/arrow,xhochy/arrow,laurentgo/arrow,icexelloss/arrow,renesugar/arrow,apache/arrow,wesm/arrow,pcmoritz/arrow,xhochy/arrow,tebeka/arrow,renesugar/arrow,pcmoritz/arrow,pcmoritz/arrow,tebeka/arrow,wesm/arrow,majetideepak/arrow,laurentgo/arrow,itaiin/arrow,majetideepak/arrow,icexelloss/arrow,xhochy/arrow,xhochy/arrow,icexelloss/arrow,itaiin/arrow,itaiin/arrow,icexelloss/arrow,icexelloss/arrow,kou/arrow,tebeka/arrow,cpcloud/arrow,laurentgo/arrow,kou/arrow,tebeka/arrow,renesugar/arrow,itaiin/arrow,xhochy/arrow,itaiin/arrow,xhochy/arrow,wesm/arrow,kou/arrow,wesm/arrow,cpcloud/arrow,majetideepak/arrow,icexelloss/arrow,renesugar/arrow,kou/arrow,wesm/arrow,renesugar/arrow,cpcloud/arrow,icexelloss/arrow,apache/arrow,kou/arrow,tebeka/arrow,xhochy/arrow,apache/arrow,itaiin/arrow,pcmoritz/arrow,majetideepak/arrow,pcmoritz/arrow,renesugar/arrow,laurentgo/arrow,itaiin/arrow,laurentgo/arrow,itaiin/arrow,majetideepak/arrow,icexelloss/arrow,tebeka/arrow,kou/arrow,kou/arrow,pcmoritz/arrow,apache/arrow,renesugar/arrow,itaiin/arrow,apache/arrow