Today, we close the connection after 10 errors. While this is fine for
normal use, it is unnecessarily large.
Lowering it to 3 helps with defense-in-depth for cross-protocol attacks
(e.g. https://alpaca-attack.com/), while still being large enough for
useful troubleshooting and normal operation.
As part of this change, we also remove the AUTH-specific failures limit,
because they're covered by the connection limit.
This patch adds a new integration test, which executes various small
dialogs, to cover corner cases that are not well covered (according to
our coverage report).
For example, "EHLO" without domain, or invalid DATA.
While we could do them via Go tests, this way is more realistic, and the
tests are easier to write.
