Consider using google-perftools or jemalloc when building MXNet

[joseph-wakeling-sociomantic]

MXNet's makefile reports:

Makefile:222: WARNING: Significant performance increases can be achieved by installing and enabling gperftools or jemalloc development packages

Assuming this won't cause any issues for dmxnet (I assume it's entirely down to how libmxnet itself works), we should try this out and see if it proves true.

[touchdown]

@joseph-wakeling-sociomantic were u able to try and see if there are perf gain with either of those two packages?

[joseph-wakeling-sociomantic]

We haven't got round to this yet, but we'll try come back to it some time in the next few months.