Add custom note locator

[Kamyki]

HI!
I found myself in need of customising ketama behaviour. From my expriences with spymemcache, number of vnodes sometimes needs fine-tuning. I also found existing issue #40 so I hope it will help others.

EDIT: better description

This pull request introduces:

• abstraction upon Conintuum class. NodeLocator interface which allows customisation of mapping of keys into memcache servers.
• builder parameter for node locator.
• simple test of custom node locator with prefix logic.

[Kamyki]

Hi maintainers,

I'm writing to bump this pull request. I have updated formatting errors pointed by last unsuccessful build. I haven't heard back from anyone yet, so I wanted to check in and see if there is anything I can do to help with the review process.

Thanks,
Kamil

[Kamyki]

Hi maintainers,
bumping PR. We are currently maintaining fork with this functionality and using it with our services, but I would gladly help with integrating it to main branch so more people can benefit from it. Just let me know what needs to be done.