Delay contract code and value updates to the end of execution

[turbolent]

Description

Until now, the behaviour of contract updates was not fully defined, there was a potential for weird side-effects.

Clearly define that a contract update, i.e. the update of contract code and contract value, is delayed until the end of execution. Contract updates shouldn't be effective immediately, as this would effectively result in hot-code patching, which is not supported.

When parsing and checking a contract code update (a new contract or a changed contract), then don't store this new program in the program storage (i.e. don't call SetProgram). Doing so would overwrite a potentially currently used program, or would result in this new program being used immediately when being explicitly imported, or implicitly imported through loading a value from storage.

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
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