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.
In this commit, we add an implementation of the recently introduced
migration.Manager interface for the address manager. With this, we'll
now be able to only expose the things required for the migration to
happen, but have the actual migration logic live at a much higher level.
The existing versions defined are set up in the same way as the existing
upgrade/migration logic, which will end up being superseded by this and
removed in a later commit.
