Recovery: be more resilient to partial network partitions

When a node is experience network issues, the master detects it and removes the node from the cluster. That cause all ongoing recoveries from and to that nodes to be stopped and a new location is found for the relevant shards. However, in the case partial network partition, where there a connectivity issues between the source and target node of a recovery but *not* between those nodes and the current master things may go wrong. While the nodes successfully restore the connection, the on going recoveries may have issues issues.

This PR adds a test that simulate disconnecting nodes and dropping requests during the various stages of recovery and solves all the issues that were raised by it. In short:

1) On going recoveries will be scheduled for retry upon network disconnect. The default retry period is 5s (cross node connections are checked every 10s by default).
2) Sometimes the disconnect happens after the target engine has started (but the shard is still in recovery). For simplicity, I opted to restart the recovery from scratch (where little to no files will be copied again, because there were just synced). To do soI had to add a stop method to the internal engine, which doesn't free the underlying store (putting the engine back to it pre-start status).
3) To protected against dropped requests, a Recovery Monitor was added that fails a recovery if no progress has been made in the last 30m (by default), which is equivalent to the long time outs we use in recovery requests.
4) When a shard fails on a node, we try to assign it to another node. If no such node is available, the shard will remain unassigned, causing the target node to clean any in memory state for it (files on disk remain). At the moment the shard will remain unassigned until another cluster state change happens, which will re-assigned it to the node in question but if no such change happens the shard will remain stuck at unassigned. The PR adds an extra delayed reroute in such cases to make sure the shard will be reassinged

I'd love it if extra focus was give to the engine changes while reviewing - I'm not 100% familiar with the implications of the code to the underlying lucene state.

There is also one nocommit regarding the Java serializability of a Version object (used by DiscoveryNode). We rely on Java serialization for exceptions and this makes the ConnectTransportException unserializable because of it's DiscoveryNode field. This can be fixed in another change, but we need to discuss how.

One more todo left - add a reference to the resiliency page




how do we deal with failures here? we just let it barf and escape?