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2023/08/04 takeshi:

I need connection_pool to run on ruby 1.8.
So I searched for a suitable gem version and it seems 2.2.3 (8b830d21fdc4c9ae1a41cc587e436478a280513c) will work with some small adjustment.

However, eventually, 2.2.3 required
  Process::CLOCK_MONOTONIC
which we don't have in ruby 1.8

But 2.2.2 uses 
  Process::CLOCK_MONOTONIC
only if available so lets try to switch to it.

So this branch was created based on 2.2.2 (608f0f407161275d95e044adad48b9ae44d55ec1).