[SPARK-4740] [WIP] Create multiple concurrent connections between two peer nodes in Netty.

It's been reported that when the number of disks is large and the number of nodes is small, Netty network throughput is low compared with NIO. We suspect the problem is that only a small number of disks are utilized to serve shuffle files at any given point, due to connection reuse. This patch adds a new config parameter to specify the number of concurrent connections between two peer nodes, default to 2.

Need to test & add test cases.

putIfAbsent returns the *previous* value, so this would be null