statement-distribution: fix parachains stalling on async_backing enablement (#3063)

Topology is coming only at the beginning of each session, so we might
lose it if prospective parachains was not enabled at the begining of the
session, so cache it for later use.

Fixes: https://github.com/paritytech/polkadot-sdk/issues/3058



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Signed-off-by: Alexandru Gheorghe <[email protected]>