ARROW-4346: [C++] Fix class-memaccess warning on gcc 8.x
ARROW-4346: [C++] Fix class-memaccess warning on gcc 8.x

Author: Wes McKinney <wesm+git@apache.org>

Closes #3467 from wesm/ARROW-4346 and squashes the following commits:

248f38c3 <Wes McKinney> Fix class-memaccess warning on gcc 8.x

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