Fix race when generation of certificate or CRL can be called from multiple goroutines

[tsaarni]

This PR fixes a race condition that is triggered when Certificate or CRL get instantiated via multiple threads using the lazy initialization method.

This happens in following scenario:

1. Global Certificate or CRL instance is created
2. Function that does lazy initialization of the object, such as Certificate.PEM(), is called from multiple goroutines. This can happen when e.g. test cases are executed in parallel and they access single shared instance of Certificate or CRL

The fix does not attempt to cover explicitly regenerating certificate and key concurrently by calling Certificate.Generate() from multiple goroutines. The reason is that it would not be possible to synchronize "accessor" calls such as Certificate.PEM() with the re-generation, so synchronization would need to be on caller side for this use case.

Fixes #47