Improve LRUHash thread safety under Ruby 2.7
Improve LRUHash thread safety under Ruby 2.7

Since https://github.com/ruby/ruby/commit/36da0b3da1aed77e0dffb3f54038f01ff574972b
Ruby checks more often for interupts in various enumerable methods,
so Hash#count for instance is no longer "atomic".

Because of this we need to syncrhonize before calling it on @table,
otherwise if multipel threads use the LRUHash, it migth fail with:

`can't add a new key into hash during iteration (RuntimeError)`

Credits goes to Alan Wu for figuring the bug.

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