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().
the manager must use a copy of the BlockListIQ's JID list, since it may
be the empty list which is not modifiable.
Also rename the getter methods from getJids() to getBlockedJids().
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
