The jids list doesn't have to be lazy initialized, because every IQ of
that type is guaranteed to contain at least one JID.
Also use ParserUtils.getJidAttribute().
Mostly remove the helper utils. The server is required to present the
client with a consisent state of the block list and corresponding
modifications, so we should not end up with duplicate entires if we
don't check for them.
SMACK-731