More StoreForward updates

[GUVWAF]

• Set heartbeat broadcast period to 15 min. instead of default_broadcast_interval_secs (which is 12 hours for ROUTER/ROUTER_CLIENT. I thought it was configurable, but it can only be disabled or enabled.
• Actually disable heartbeat when configured; this->heartbeat was not used. I'm planning to open a PR for the docs to add a tip to disable it once all clients identified the Store & Forward Router, since it's not needed afterwards.
• Actually check whether historyReturnMax is not yet reached (right now it could cause a buffer overflow).
• When PSRAM is full, start overwriting it. Still not ideal as you might miss messages that were added at the end, but at least better than doing nothing anymore once you reach it.
• Require "SF" to be sent as DM to the Store & Forward Router to avoid multiple sending the history at the same time if there are multiple on the mesh.