OOM DoS resistence measures from C tor

As part of our work on onion services in arti, we should provide some or all of the same OOM DoS mechanisms as used in C tor.

Subtasks:

• Memory DoS:
  • Track memory consumption for each stream and circuit.
  • Track total memory consumption for all streams and circuits.
  • Track age of data on streams and circuits.
  • Configure and/or guess a memory limit.
  • Code to notice when we're low on memory and kill streams and circuits.

Outstanding work before calling this done for how

• #1660 (closed) internal architecture docs improvements !2509 (merged)
• #1661 (closed) abolish separate StreamAccount !2505 (closed)
• #1659 (closed) avoid accidentally making untracked queues
• add to example config file
• compile it in by default (but disabled - no quota) !2569 (merged)
• whatever ad-hoc test we can manage (at least, check that the config works and the memquota system is engaged) -- enabled in shadow test in !2560 (merged)
• check that there are no TODO #351 remaining in tree. Done.

References

• dos-spec.md
• tor-spec section 8.1
• The "sniper attack" paper by Jansen, Tschorsch, Johnson, and Scheuermann.