Feature Request: store mheard in external DB

[mdomsch]

The current (1.5) implementation of the recently-heard station list (mheard) stores the stations in an in-memory hash table. This means the table is effectively emptied at process stop/start.

When igating from SMSGTE to RF, direwolf looks up in mheard if it has heard the station "recently" (e.g. within the last hour).

This opens up the opportunity for an ongoing APRS/SMS conversation to get "broken" when direwolf is restarted, as direwolf won't have a record of the RF station until it transmits again.

Would it be feasible to implement the mheard state within an external database that's used, such as sqlite, that exists across direwolf restarts?