Catchup needs to be finished during high load

As was found in INDY-1404, if a lot of catchup-related messages (LEDGER_STATUS, CONSISTENCY_PROOFS) are lost, the node doesn't try to start catchup again.
The messages can be lost due to high load (as in the test), and limited size of node-to-node's message queue.

The recent fixes regarding catchup and fixes done in the scope of INDY-1404 made the problem more visible, since we require that the previous round of catchup is finished before starting the next round.

Acceptance criteria:

• The following tests need to be written:
  • Skip sending of Message Responses to LEDGER STATUS request once for domain ledger to a node doing catchup; make sure that the node eventually finishes catchup
  • Skip sending of CONSISTENCY_PROOFs for domain ledger once to a node doing catchup; make sure that the node eventually finishes catchup
• A Node must finish catchup regardless of the load in the pool
• Catchup can be finished with some delay (in a timeout),