use loadbuffer and deprecate load

[renefloor]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bugfix for #772

⤵️ What is the current behavior?

🆕 What is the new behavior (if this is a feature change)?

💥 Does this PR introduce a breaking change?

No, only in the platform interface

🐛 Recommendations for testing

📝 Links to relevant issues/docs

#772

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[codecov]

Codecov Report

Base: 95.67% // Head: 82.14% // Decreases project coverage by -13.53% ⚠️

Coverage data is based on head (f611c96) compared to base (607a2eb).
Patch coverage: 74.28% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #773       +/-   ##
============================================
- Coverage    95.67%   82.14%   -13.54%     
============================================
  Files            4        5        +1     
  Lines          162      196       +34     
============================================
+ Hits           155      161        +6     
- Misses           7       35       +28     

Impacted Files	Coverage Δ
...e/lib/cached_network_image_platform_interface.dart	50.00% <33.33%> (ø)
...rk_image/lib/src/image_provider/_image_loader.dart	88.00% <75.00%> (-6.74%)	⬇️
.../image_provider/cached_network_image_provider.dart	56.36% <79.16%> (-43.64%)	⬇️

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