Obfuscate the attachement download links in debug log

[breznak]

As discussed here #1442 (comment)
The log contains links for attachment download which look like:
https://whispersystems-textsecure-attachments.s3-accelerate.amazonaws.com/3399142333737380479?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Date=20180103T223203Z&X-Amz-SignedHeaders=content-type;host&X-Amz-Expires=3600&X-Amz-Credential=AKIAJHWS3AOTJTASHBDA/20180103/us-east-1/s3/aws4_request&X-Amz-Signature=e1e8f3ac9782447d120430b2747fbe4cf067262a33302ae9cc60ee02a6e29b97

Regarding the attachments, the links expire over time, and the actual file contents are encrypted. But you're right, we could probably obscure some of those querystring arguments to make it even harder to get access to those attachments.

Suggested redaction:
https://whispersystems-textsecure-attachments.s3-accelerate.amazonaws.com/3399142333737380479?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Date=20180103T223203Z&X-Amz-SignedHeaders=content-type;host&X-Amz-Expires=3600&X-Amz-Credential=REDACTED/20180103/us-east-1/s3/aws4_request&X-Amz-Signature=REDACTED9b97