Fix clear token

All the current implementations of TokenStorageInterface::clearToken() seem to clear all tokens, not just one single service token, despite the comments and the method name leading one to think that only one token is deleted.

This patch fixes that by renaming the current `clearToken` methods to `clearAllTokens`, and adding a new `clearToken`method that only clears the specific service token.

Fixes https://github.com/Lusitanian/PHPoAuthLib/issues/75.