RFR: 8245024: Simplify and eagerly initialize StringConcatFactory

[Claes Redestad]

Hi,

JDK-8155090 added some measures to StringConcatFactory to prevent
certain bootstrapping issues. I think we should consider eagerly
initializing the StringConcatFactory base class during bootstrap
instead.

Doing so will add a tiny bit of class loading overhead to bootstrap
(which of course will be amortized on first use of SCF) but still this
would also be a good opportunity to clean up a little.

One straightforward means to do so is to move the CACHE, DUMPER and
CACHE_ENABLE fields to BytecodeStringBuilderStrategy, since it's the
only place where they are used (the MethodHandle based strategies don't
manually spin classes and rely on LF sharing to de-duplicate shared
components).

Webrev: http://cr.openjdk.java.net/~redestad/8245024/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8245024

This reduces initialization overhead and simplifices bootstraps for the
default case, while still passing relevant test cases such as test/jdk
/java/lang/String/concat/WithSecurityManager.java for all strategies.

Testing: tier1-2

Thanks!

/Claes