Replace zap with zerolog.
zerolog has a cleaner interface and can be easily configured with custom
error chain printing using a new error handling library that will be
implemented in another PR.
Just a raw replace of "github.com/pkg/errors".
Next steps will improve errors (like remote errors, api errors, not exist errors
etc...) to leverage its functionalities
* Rename to datamanager since it handles a complete "database" backed by an
objectstorage and etcd
* Don't write every single entry as a single file but group them in a single
file. In future improve this to split the data in multiple files of a max size.
