wallet/chainntnfs: add sanity check for birthday block before syncing
wallet/chainntnfs: add sanity check for birthday block before syncing

In this commit, we add a sanity check for the wallet's birthday block
before syncing as a result of the migration that populated it for
existing wallets. This is done as the second part to the migration to
ensure we do not miss any relevant events throughout rescans.

The sanity check performs two main checks: whether the birthday block
timestamp reflects a time before the birthday timestamp and whether the
delta between these two timestamps is a reasonable amount. The birthday
block is then found as the first candidate that satisfies both of these
conditions.

btcsuite/btcwallet,btcsuite/btcwallet