- rename private methods
- inline private on-shot methods
- extract common local var
- add more final modifier for vars

- TTL do NOT have the ownership of thread local/snapshot!
- more gc friendly, avoid potential memory leak!

- use WeakHashMap type for implementation
  express the weak semantics implicitly
- use keySet instead of entrySet
  simplify the code

JSR305 is already Dormant status, so SpotBugs does not release jsr305 jar file.
Please continue using findbugs’ one.
depend on spotbugs-annotations instead.

https://spotbugs.readthedocs.io/en/stable/migration.html

disable Inheritable feature by using InheritableThreadLocal
can avoid potential leaking problem described in #100

more discussion see #95

- build by java 8 with java 6 source/target
    - full functions with java 8 support(ForkJoinPoolTest)
    - support java 6
- Travis ci has multiple java version: java 6/7/8/9
- scripts and test case/demo cleanup