Include history refs in Group::write()

[simonask]

This should enable offline file-compaction of sync'ed Realms (both on client and server).

[codecov-io]

Codecov Report

Merging #3039 into master will increase coverage by <.01%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master    #3039      +/-   ##
==========================================
+ Coverage   93.08%   93.08%   +<.01%     
==========================================
  Files         270      270              
  Lines       78945    78971      +26     
==========================================
+ Hits        73487    73514      +27     
+ Misses       5458     5457       -1

Impacted Files	Coverage Δ
src/realm/group.hpp	97.67% <100%> (+0.02%)	⬆️
src/realm/table.cpp	90.79% <100%> (ø)	⬆️
src/realm/group.cpp	93.56% <95.65%> (+0.06%)	⬆️
src/realm/column_table.cpp	83.33% <0%> (-0.11%)	⬇️
test/test_thread.cpp	82.64% <0%> (+0.34%)	⬆️

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[realm-ci]

Check the performance result here: https://ci.realm.io/job/realm/job/realm-core/job/PR-3039/1/Performance_Report

[finnschiermer]

Looks good to me :-D

[realm-ci]

Check the performance result here: https://ci.realm.io/job/realm/job/realm-core/job/PR-3039/2/Performance_Report