JDK-8155090 added some measures to StringConcatFactory to prevent issues with a security manager that does string concatenation. As an alternative approach, we should consider eagerly initializing the StringConcatFactory base class during bootstrap.

Doing so will add minute overhead to bootstrap - which will be amortized on first use of SCF. This would also be a good opportunity to clean up and improve the initialization.

For example, the CACHE and CACHE_ENABLE fields can be moved to BytecodeStringBuilderStrategy, since it's the only place where we actually have implemented caching (the MethodHandle based strategies rely on LF sharing to de-duplicate shared). This reduces the initialization overhead of SCF for the default case.