Faker instance is not thread-safe

[AlekSi]

Faker instance can't be used by different goroutines in parallel tests (t.Parallel()), there is a data race.
I propose to make it thread-safe, since creating new instance in every test is a waste of resources.
Alternatively, explicitly document that it's not thread-safe.

[AlekSi]

https://github.com/enodata/faker is thread-safe.