Log callback functions

[fasmat]

This integrates the changes from spacemeshos/post-rs#47 to allow postrs to log while being used in go-spacemesh.

[poszu]

I don't think that GenerateProof() and others should accept logger to be set for logging in the post-rs library. Only the first one will be set and remembered forever. While it's probably not a big deal in application code (as there will only be a single logger forever), in tests it will cause all logs to be recorded in the logger created for the first test that was executed probably causing them to appear in the wrong place (?).

I was imagining that there could be an exported function func ConfigureLogging(logger *zap.Logger) documented to be idempotent, which should be called once by the application on its boot (starting phase of postcli or go-spacemesh).

[fasmat]

I think a global logger in general is an issue. A ConfigureLogging function is a workaround for the main application, but our approach to logging is not to have one global instance but (possible multiple) instances that are dependency-injected into the services.

With a global logger running tests in parallel becomes more difficult because logs from different tests are all printed to the same output concurrently and cannot easily be associated with the test they came from. This is why for instance zaptest.NewLog takes an instance of testing.T, so that the logs can be sorted to the tests they came from and go test can print the outputs correctly.

EDIT: and tests can have different logging levels (e.g. tests where logs are irrelevant have a no-op logger, some have only info for dependent services and debug for the service at test, etc.)