HTTP: Drop exceedingly large requests on the floor

We can't send a response until we have all the data, so this caused an
assert with the included test case.
Add more buffer tests

The reason the debuglog() was added in /echo was that we need to test the
IPC with large data. There used to be a bug which was triggered when
multiple read()'s were required to fetch the data. This /echo-test should
test that code now.