Add `start_height` parameter to `bitcoind_rpc::FilterIter::new`

[luisschwab]

From evanlinjin's comment on bdk_wallet#368:

bitcoind_rpc::Emitter::new has a start_height parameter. This is used in the case the closest block that connects is below the start_height. So if the tip is reorged out, and we only have tip + genesis, we will start from start_height instead. To convey the wallet birthday with the checkpoint tip, while still taking into account reorgs, we can use tip_height - FINAL_DEPTH as the start_height parameter. FINAL_DEPTH can be as conservative as you want.

bitcoind_rpc::FilterIter::new does not have such a parameter. We can add it?

Then we provide docs as how to convey the wallet's birthday to the chain source using the checkpoint tip.