This patch introduces stress tests, which put load on the daemon to help
sanity check its behaviour under stress.
They are separate from the existing integration tests, which focus on
correctness.
Two tests are included here: a load test, which sends emails repeatedly;
and a connection test which opens as many conections as possible.
This patch implements some measures against email loops, such as keeping
a limit on the lenght of an address, and rejecting email that has too
many Received headers.
It's not perfect (a server could be actively removing Received headers),
but it should cover the normal accidents and misconfigurations.
