mnesia: Check nodes after protocol negotiation

During Mnesia startup, after protocol negotiation, the list of connected
nodes is written to "recover_nodes". This list is later used to merge
the schema.

If Mnesia was stopped on a remote node between the protocol negotiation
and the moment the list is stored in "recover_nodes", the remote node
is still considered running: the value of "recover_nodes" stored during
mnesia_down/1 is overwritten. Therefore, this node may be used to
acquire a write lock on the schema in order to perform the merge. In
this case, the remote node never answers to the lock request and Mnesia
hang forever (application:start(mnesia) never returns).

To fix the problem, we check the list one last time and remove from it
all nodes where Mnesia is stopped. And because there is still a chance
for missing mnesia_down event, handle_cast({mnesia_down, ...}, ...)
writes to recover_nodes again, in addition to mnesia_down/1.