Enable extensive golangci-lint checks

Adds a golangci linter configuration with almost all linters enabled. Some deprecated ones are disabled.
Further, some linters are not suitable for test files and therefore deactivated in *_test.go files. See `.golangci.yaml` for details. The version of golangci-lint is pinned to 1.45.2.

Changes to the source code are applied to make the linter happy in the first place. Some changes need further refactoring (e.g. error handling) and sometimes linter warning are disabled by 'nolint' comments. Addressing those will be part of further refactoring.

Signed-off-by: Jens Drenhaus <jens.drenhaus@9elements.com>