Print warnings when fetching pending blocks (#8711)

* Lots of println to figure out what eth_getBlockByNumber does/should do

* Remove debugging

* Print warnings when fetching pending blocks

When calling `eth_getBlockByNumber` with `pending`, we now print a deprecation warning and:

* if a pending block is found, use it to respond
* if no pending block is found, respond as if if was a request for `Latest`

Addresses issue #8703 (not sure if it's enough to close it tbh)