Report events in the background

[vmalloc]

• Detect existence of necessary APIs, fall back to sync mode
• Implement a background queue
• Correct all time-sensitive APIs to report original time
• Implement proxy objects for tests and sessions
• Need to sync with the server upon initial discovery about server time
• Support multiple Backslash instances at once

[vmalloc]

Blocked by getslash/backslash#449