More robust s3 reader
More robust s3 reader

Merge the following dmlc improvements to s3 reader:
- [S3] more robust reader
  https://github.com/dmlc/dmlc-core/commit/8e0c5a72d99436e158c262447a01ee7993bf3e81
- [S3] use recommend default timeout, and print an error msg when retry
  https://github.com/dmlc/dmlc-core/commit/c4555165eff587a1fc240e4d252c30855542ffca

TobyRoseman/SFrame,dato-code/SFrame,TobyRoseman/SFrame,hoytak/SFrame,dato-code/SFrame,dato-code/SFrame,ylow/SFrame,ylow/SFrame,dato-code/SFrame,turi-code/SFrame,turi-code/SFrame,TobyRoseman/SFrame,TobyRoseman/SFrame,hoytak/SFrame,ylow/SFrame,turi-code/SFrame,hoytak/SFrame,hoytak/SFrame,ylow/SFrame,ylow/SFrame,hoytak/SFrame,TobyRoseman/SFrame,hoytak/SFrame,dato-code/SFrame,turi-code/SFrame,hoytak/SFrame,ylow/SFrame,dato-code/SFrame,turi-code/SFrame,dato-code/SFrame,TobyRoseman/SFrame,dato-code/SFrame,ylow/SFrame,TobyRoseman/SFrame,turi-code/SFrame,hoytak/SFrame,turi-code/SFrame,ylow/SFrame,turi-code/SFrame,TobyRoseman/SFrame