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

- TtlRunnable
- TtlCallable
- ExecutorTtlWrapper
- DisableInheritableThreadFactory
- DisableInheritableForkJoinWorkerThreadFactory

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

- 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

add TransmittableThreadLocal.Transmitter, expose internal capture/replay/restore operations for TTL values

1. Transmitter#capture: captures TTL values in current thread
2. Transmitter#replay: replays the captured TTL values after back up
the current ones
3. Transmitter#restore: restores current thread to state before replay

- update code and doc: concept multi-thread context(MTC) => TTL
- package mtc => ttl
- update word 'context' to 'TTL value'