Fix crash from accessing deleted SubtreeFileReader

[pjanetzek]

This change keeps SubtreeFileReader instance in the loaders future context. Should fix #774

[j9liu]

Thanks @pjanetzek! Before we review your code, can you sign a CLA (contributor license agreement) here?

[pjanetzek]

Hi @j9liu as far as I know Peer signed the CLA for our team - could you please check if my account is on your list?

[kring]

Yes we have a CLA for you, thanks @pjanetzek!

[kring]

Thanks @pjanetzek! I pushed a few small changes:

• Renamed a variable to match our conventions.
• Modified a test to make it catch this bug. We have decent tests for availability subtree loading, but unfortunately the process is much more synchronous in the tests, so it didn't trigger this crash.
• Added some documentation of this requirement that SubtreeFileReader remain valid until the returned future completes. I considered ways to eliminate this requirement, because it's not very intuitive, but the cost of every solution I could think of outweighs the benefit.
• Updated CHANGES.md.