feat(mria_membership): notify monitoring processes when mria is down

[SergeTupchiy]

The event will be sent in the following cases:

1. A replicant node detects that another (either core or replicant) node is down.
2. A core node detects that a replicant node is down.

Core nodes still track each other only at Mnesia level (by subscribing to Mnesia system events).

An exception to the above is when Mria is down on a local node. In this case mria_membership will notify its monitoring processes before terminating.