cactoos: SolidScalar is slow

SolidScalar uses syncronized access to cached value, that is not really fast in comparation with mutable volatile fields (~ 50 times faster). I created an app to prove this: https://github.com/g4s8/synctest I compared SolidScalar vs double check + volatile scalar and got these results for 10_000_000 executions on 17 threads: (values are average of per vallue() execution) SolidScalar - 2400 - 3200 nano-seconds double-check scalar - 58 - 61 nano-seconds