Merge model compatibility fixes from 1.0rc branch.

[trivialfis]

#5281
#5248

[mli]

Codecov Report

Merging #5305 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5305   +/-   ##
=======================================
  Coverage   83.76%   83.76%           
=======================================
  Files          11       11           
  Lines        2409     2409           
=======================================
  Hits         2018     2018           
  Misses        391      391           

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[trivialfis]

The only conflicting file is Learner.cc where both branches modified Copyright year. So it should be safe to merge this PR.

All testing code are also merged here.