Bad performance locking

actually this code is not parallel at all because of the `synchronized` keyword, I suggest using `reentrant lock`, wdyt?

https://github.com/yegor256/cactoos/blob/9ee0dd1fef855129104fdbbf6e66d6eafd4bfb05/src/main/java/org/cactoos/scalar/Synced.java#L82