Fix session migration and assign message history to correct sessions

[Blackoverflow]

This one contains:

• a migration for the missing sessions
• a migration to correctly assign the message history to the new sessions (leaving the old sid in a new column sv1id)
• a update of signal-golang/textsecure to version 1.19.0

Still to do:

• cleanup
• verification that this works like intended
• reference new release of textsecure

[olof-nord]

I know it is work in progress, but if we are thinking of switching to a fork of textsecure lets clarify the reasons why and start to think of how to upstream our changes?

[nuehm-arno]

Will do a review tomorrow!

[nuehm-arno]

I tried to review, but the changes are a bit too much for my limited go knowledge - sorry.