fuzz_task: Migrate crashes to uworker.

[oliverchang]

• Add proto definitions for crash groups.
• Have uworkers return serialized CrashGroups, with the input testcases uploaded via signed URLs.
• Handle checking if crash is new, testcase creation etc in postprocess.
• Remove old uworker_msg.CrashInfo conversion and move the job_run crash group computation to postprocess, as it relies on querying the Datastore to know if the crash is new or not.
• Also handle uploading file handles in storage.upload_signed_url
• Add bot_name, platform_id to uworker output.

TODO: tests.