Fix race condition in mnesia_locker after nodedown for sticky_locks

Situation:
* node A and B have copies of table T
* node A ows sticky of table T
* node A goes down (e.g. crash)
* node B tries to perform transactional operation on table T
  (e.g. mnesia:select)

In this situation there is possibilty that first (and maybe other)
transaction on node B will hang indefinitely.
This is caused by race condition, when transaction process send lock
request operation to node A and waits for reply. When node A is down
it will never send reply, so process on node B will be stucked
forever.
Reason is that message sent to mnesia_locker gen_server from
mnesia_locker:mnesia_down can be received after mnesia_locker gen_server
already replies to transaction processes with {switch, N, Req} and
node N is down.
Monitoring remote process when sending request to other node should
be safe soltuion.