Flush kernel logs to channel on exit

[lionel-]

Addresses #707.

logTail doesn't seem to have any way to flush the log file so I implemented it manually in the dispose() method.

[jmcphers]

Why does dispose and disposeLogTail need to be async? (I didn't see any new async calls in the function bodies, but maybe I missed them...)

[lionel-]

Why does dispose and disposeLogTail need to be async? (I didn't see any new async calls in the function bodies, but maybe I missed them...)

The readline iterator over the lines is async, so we iterate over the lines with for await.