Dont save sent MessageStanza for LMC use

There are mutliple reasons why this is not a good idea

1. It places work on encryption Plugins, as many encryption attributes inside the
stanza can not be resend again (OMEMO, OTR), so the plugins have to make sure none of
these attr are inside the LMC stanza

2. In general its not obvious for plugin devs that a stanza issued after LMC has to be
treated differently. There should be no negative effects, even when a contributor not knowing
about LMC at all.

This commit saves only the stanza id, and adds the replace tag on the new message.
This results also in less code.