rocker: added back thread-local buffer cache

[jknack]

It was removed in 3.x, but we need it for be on top of techempower benchmarks (like all the other java web frameworks)

[agentgt]

@jknack If you are interested I can make the techempower benchmark use JStachio instead of Rocker.

Multiple projects have shown JStachio is pretty much the fastest templating language in Java but of course I'm biased.

[agentgt]

@SentryMan has an example of it being used in TechEmpower for avaje-jex:

https://github.com/TechEmpower/FrameworkBenchmarks/blob/master/frameworks/Java/avaje-jex/src/main/java/benchmark/model/FortuneTemplate.java

https://github.com/TechEmpower/FrameworkBenchmarks/blob/master/frameworks/Java/avaje-jex/src/main/resources/fortunes.mustache

We will need to had the threadlocal buffering we took out of the JStachio module.