Export ensurePerformance so PLATFORM.performance can be used without initializing jsdom

[timfish]

aurelia-store uses PLATFORM.performance to time and output performance data. When using the store in nodejs, the full aurelia-pal-nodejs initialize(), results in 6MB of JavaScript being bundled including jsdom.

This change should allow us to init PLATFORM.performance on nodejs without bringing in all the other dependencies.

Related aurelia-store issue:
aurelia/store#66

[EisenbergEffect]

@timfish Was startoffset just an old, unused variable that never got cleaned up as far as you can tell?

[timfish]

@EisenbergEffect yes, startoffset wasn't used so I took the opportunity to remove it.

It looks like perf_hooks might be able to replace lots of this when node v10 becomes more mainstream: https://nodejs.org/api/perf_hooks.html

[EisenbergEffect]

Thanks @timfish ! Also, I'd love if you could share a bit with @zewa666 about how you're using Aurelia Store in Node.js. I'm sure the use case would help him as he continues to build Aurelia Store and as we port it to Aurelia vNext as well.

[zewa666]

We had already a fair amount of conversation. This PR being a part of putting the plugin on a diet