ARROW-4546: Update LICENSE.txt with parquet-cpp licenses
ARROW-4546: Update LICENSE.txt with parquet-cpp licenses

- Ported parquet-cpp external license references
- Removed spurious duplicates (boost, mapbox)

Author: François Saint-Jacques <fsaintjacques@gmail.com>

Closes #3692 from fsaintjacques/ARROW-4546-parquet-license and squashes the following commits:

a5aa81e48 <François Saint-Jacques> ARROW-4546: Update LICENSE with parquet-cpp licenses

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