Feature/optimize cache

[KIwabuchi]

This PR is for a completely new object cache implementation.

The new cache is:

• Uses much less memory
• Significantly faster initialization time, especially on a machine that has many CPU cores
• More compact and packed internal data structure
• Yet, achieves the same Metall performance on the dynamic graph construction workload

Simple create and close test results (just creating and closing Metall manager).

• New ver
  Create 0.0440739s
  Close 0.00165344s

• Old ver
  Open 0.658001s
  Close 0.116477s