In this commit, we add a new migration to the waddrmgr to populate the
birthday block for existing wallets. This will deem useful when
performing rescans for whatever reason, as we'll now be able to start
from this point rather than the genesis block, incurring a longer
rescan.
The migration is not as reliable since we do not store block timestamps,
so we'll need to estimate our height by looking at the genesis timestamp
and assuming a block occurs every 10 minutes. This can be unsafe, and
cause us to actually miss on-chain events, so a sanity check will be
added before the wallet attempts to sync itself in a later commit.
