[FLINK-2882] [core] Improve performance of string conversions

[greghogan]

Memoize string representations of AbstractID.

Use lookup table for byte-to-hex conversion in StringUtils.

[tillrohrmann]

Thanks for your contribution @greghogan. The efficiency improvements look really good. The test failures seem to be unrelated.

Would be great if you could add a small test for the StringUtils method.

Apart from that, +1 for merging.

[StephanEwen]

Looks very good.

I am wondering if we can get rid of the toShortString method. What is that one actually used for?

[uce]

Yes, that's good to remove. I pointed that out as well when Greg opened the initial issue.

We can either do it as part of this PR or I can do it as a follow up.

[greghogan]

@tillrohrmann There exists StringUtilsTest with small test.

One of the TravisCI errors was a timeout and the other killed by the watchdog. Are timeouts being reworked to use RetryOnException or similar?

@uce It looks like toShortString is used for more than debugging and tests so I'd go for a separate PR.

[fhueske]

Looks good. 1+ for removing toShortString in a separate issue.
I'll merge this PR.