Scanner should log a message when Zebra is before the Sapling activation block

[teor2345]

I think we should print a message as soon as we know we are behind the sapling activation block saying something as:

"scan task will not get started until the blockchain is at sapling activation height (block number: {sapling activation_height})"

This way users will know the scanner will do nothing until that.

We can look for that message in this test so it will not require further modifications. A new test will be done with state.

Originally posted by @oxarbitrage in #8040 (comment)