If the load generator is sending emails too fast, chasquid queue might
hit the maximum size and fail the test.
This patch makes it sleep and retry, to give the server some time to
catch up.
Thanks to Max Mazurov (fox.cpp@disroot.org) for reporting this problem.
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.
