Track last blocks in informant display

This implements tracking of the last seen blocks in informant display
to prevent printing the import message twice. In Cumulus we first import
blocks as part of the block building with `new_best == false` and set
the best block after we know which one was included by the relay chain.
This leads to printing the import messages two times. This pr solves the
problem by track the latest seen blocks to not print the message twice.