fixed high memory usage

[pk910]

I've implemented a fix for #39 as I ran into the same problem.
The underlying problem for this high memory consumption is, that nodejs only runs its garbage collector as a subtask.
This subtask is never executed when running a endless loop.

To resolve the issue I've changed the worker implementation to run via a interval instead of a blocking while(true) loop.
The VanityEth.getVanityWallet now stops after a given amount of time, so it's not blocking forever, but gets recalled from the interval almost immediately.

I'm running it on a server for a few hours now.
It's consuming max CPU, but doesn't go above ~70MB memory anymore :)

[J9B10]

Excellent!
It worked really well here.
Now fixed memory overflow.

I thank.

I suggest a change:
Change the time interval from 1000 to just 10.
This improves execution speed dramatically.

[tansanDOTeth]

This worked well for me

[tansanDOTeth]

Excellent! It worked really well here. Now fixed memory overflow.

I thank.

I suggest a change: Change the time interval from 1000 to just 10. This improves execution speed dramatically.

I needed this for something, so I added a PR wth both the changes by @pk910 and your suggestion.