lib/babe: first slot of network should be set on finalization of block 1

[noot]

Describe the bug

• the first slot of the network is determined by when block 1 was built
• however gossamer currently sets the first slot of the network in 2 places (if an authority node), when just after building block 1 OR when importing block 1. this potentially results in being unable to validate imported blocks or the BABE algorithm becoming un-synced from the network as the first slot is set incorrectly
• the first slot number should be set on finalization of block 1, instead of build/import
• in the case where block 1 has not yet been finalized, there should be some list of unfinalized blocks with block number 1. when verifying a block and block 1 is not yet finalized, it should first use the slot number of block 1 on the canonical chain (which is what currently happens) as the first slot, but if that errors, try using other pending block 1.

sub-issue of #1587