Populate topology after restart if finality is lagging behind current session (#6913)

There is a small issue on restart, where if finality is lagging across
session boundary and the validator restarts, then the validator won't be
able to contribute anymore with assginments/approvals and gossiping for
the blocks from the previous session, because after restart it builds
the Topology only for the new session, so without a topology it won't be
able to distribute assignments and approvals because everything in
`approval-distribution` is gated on having a topology for the block.

The fix is to also keep track of the last finalized block and its
session and if it is different from the list of encountered sessions,
build its topology and send it to the rest of subsystems.

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Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>
Co-authored-by: ordian <write@reusable.software>