not_computed context

[jerch]

This PR implements a very basic context to disable resolver updates temporarily. For more details see #148.

Note that there is no auto-recovery from the desync state implemented yet, so it is fully the user's responsibility to do that afterwards.

TODO:

• write some docs
• thread isolation of disabled state

Shall fix #148.

[coveralls]

Pull Request Test Coverage Report for Build 16175036780

Details

• 57 of 57 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 95.043%

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Totals
Change from base Build 16121043416:	0.1%
Covered Lines:	1181
Relevant Lines:	1227

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💛 - Coveralls