[Ovmsdev] Revised memory diagnostics code

[Stephen Casner]

WARNING!  I have revised the memory diagnostics code in the app and in
esp-idf such that both need to be updated at the same time if you have
enabled CONFIG_OVMS_DEV_DEBUGRAM and CONFIG_OVMS_DEV_DEBUGTASKS.

See the commit messages for details.

                                                        -- Steve