Don't generate poison keys during the query

Right now every call to the GetPoison* creates keys, if they don't exist. These results in a superfluous side effect, unexpected behaviour and delays in decryption (because we try to check records with fresh keys, which obviously do nothing useful).

Change this, by:

Introducing new interface, which is responsible for poison key generation
Removing creation of keys in Get methods

Also, keep this behaviour for poisonrecordmaker, but do this explicitly.

Checklist

[x] Change is covered by automated tests
[x] The coding guidelines are followed
[x] Public API has proper documentation in the Acra documentation site or has PR on documentation repository with new changes: here.
[x] ~CHANGELOG.md is updated (in case of notable or breaking changes)~
[x] CHANGELOG_DEV.md is updated
[x] ~Benchmark results are attached (if applicable)~
[x] ~Example projects and code samples are up-to-date (in case of API changes)~

cossacklabs / acra

Don't generate poison keys during the query #516

Checklist