MXNET-1294: Priority-based parameter propagation for improved data parallel training throughput

[Anand J]

Hi All,

I'm planning to add some improvements to MXNet KVStore based on the 
ideas from my recent SysML'19 paper
https://anandj.in/wp-content/uploads/sysml.pdf. I have created a JIRA 
ticket on this: https://issues.apache.org/jira/browse/MXNET-1294.

The code changes I'm planning to do require changes in PS-Lite and 
KVStore. I'm new to MXNet developer community. It would be helpful If 
someone can give guidance on how should I be proceeding with this.

Thanks,
Anand