Support to dynamically teardown Tenants

[Xzelsius]

Is your feature request related to a problem? Please describe.
With the added support for dynamically adding tenants and spin up their agents (which is great 💯btw) we face an issue when we tear down a tenant at runtime.

All connected agents for said tenant will continue to run and try to poll something from the database.
But since the database was removed from the master table and after that, the database was dropped, it will result in

Npgsql.PostgresException (0x80004005): 3D000: database "xxx" does not exist
  Exception data:
    Severity: FATAL
    SqlState: 3D000
    MessageText: database "xxx" does not exist
    File: postinit.c
    Line: 945
    Routine: InitPostgres

And lots of them :)

Describe the solution you'd like
It would be beneficial if the mechanism that determines the availability of a new tenant to initiate its agent could also identify when one of its active agents is no longer present in the master table and consequently terminate that tenant's agent.

Describe alternatives you've considered
If the above is rather complicated to implement, one other solution could be to detect the 3D000 error within the agent and stop it after a few retries.

Additional context
In our SaaS solution we dynamically provision tenants at runtime.

If a new tenant is created, we do

1. Create a new entry in the Master Table using Marten's Tenancy
2. Create the Postgres Database manually
3. Use Marten's DB Schema Migration functionality to create all DB objects
4. Seeds some system data using Marten

If a tenant is deleted, we do

1. Remove the entry from the Master Table using Marten's Tenancy
2. Drop the Postgres Database manually

Our solution consists of 4 different Wolverine powered applications.
Each application may have multiple replicas. They all work with the same Master database, but have their individual Schemas for Wolverine.

Tenants have their own Inbox/Outbox within their tenant database.