Stricter validation of connect messages

Previously, you could leave out the "support" field (or claim to not
support the version you were proposing) which would mean your connection
would succeed iff the proposed version is the server's favorite
version. This led to (eg) ObjectiveDDP accidentally writing a client
that stopped working when servers started preferring pre2 over pre1. By
making this a blatant error, DDP client libraries are more likely to
be written in a way that works with version negotiation.

Also, remove the delay in sending connect failure messages, which was
intended to avoid connect storms from clients that are by now 1.5 years
old.

Fixes #2125.