Because of limitations in the jsonrpsee API, it's not possible to log HTTP
request information and the actual JSON-RPC payload in one log message; but
the two log messages should appear right next to each other.

Logging both together would require a more complex upgrade to a newer
jsonrpsee version.

If we didn't have an entry for key in last_mod, we would immediately return
with None instead of searching for the key in the fallback part of the
method.

This also tries to clean up the code a little by introducing a LastMod
struct that encapsulates different behavior between having batch
memoization turned on and turned off.

Fixes #6313