Refactor first sync with Synapse

[fredo]

The current implementation of initialize_first_sync() in MatrixTransport has the purpose to get the inventory of rooms for our partners. With the removal of rooms this is not needed anymore. Instead there might be a longer list of messages stored for the user which can contain lots of call messages for webrtc connection attempts.
Since the user will try to create a web rtc connection himself with its partners we can ignore those messages.
It is also possible to ignore all messages in the mailbox as we have the retry mechanism in place and Raiden protocol messages will be retried anyway.

• Remove the filter of not fetching messages
• perform /sync manually (as currently already done)
• dump all the messages

[fredo]

This is also partly covering #6824