Teardown gracefully on MemoryError

[abhinavsingh]

Reproducible by running proxy.py on smallest GCP instance (or a system with <500Mb of free RAM) and stress testing it via fast.com speed tests.

Traceback (most recent call last):
File "./proxy.py", line 1082, in run
  teardown = self.run_once()
File "./proxy.py", line 1067, in run_once
  teardown = plugin.read_from_descriptors(readable)
File "/home/mailsforabhinav/proxy.py/proxy.py", line 834, in read_from_descriptors
  self.response.bytes += raw
MemoryError

[abhinavsingh]

Marking as enhancement / feature request since this is not a disruptive bug.