XEP-0198: Don't warn on invalid ACK elements

Do not log a warning (but only a debug message) if the client sends an
invalid </a> packet.  Some clients do that occasionally, and there's
nothing server admininistrators could do about that.
XEP-0198: Reject <resume/> with negative 'h' value

Make sure the 'h' attribute sent with a <resume/> request is
nonnegative, as mandated by XEP-0198.

We already have this check for <a/> elements.
XEP-0198: Improve handling of too large 'h' values

If the client says that it handled more stanzas than we sent (due to a
bug in the client's or in our code), increase our outgoing stanza count
accordingly.  There's no point in sticking to the old value even if it
was correct, as the client surely won't fix its count during the current
session.
Merge pull request #205 from weiss/xep-0198

XEP-0198: Improve handling of incorrect stanza counts reported by client