Improve Verification Aborts

[ArquintL]

As observed in Viper-IDE #370, a subsequent verification might be affected by an earlier abort. We have observed that ViperServer keeps sending messages to Viper-IDE even though the abort has finished. The probable reason is that the propagation of messages is delayed. Thus, this PR performs these two contributions:

• skip all messages after performing an abort such that a client does not confuse them belonging to a potential subsequent verification job
• abort not just the verification job but also the AST building job

[ArquintL]

bors merge

[bors[bot]]

Build failed:

• test - windows-latest

[ArquintL]

bors retry

[bors[bot]]

Build succeeded:

• check-license-headers
• test - macos-latest
• test - ubuntu-latest
• test - windows-latest