Do not cache anything when max capacity is zero

[tatsuya6502]

To fix #230, make the cache implementations to store nothing when the max capacity is set to 0.

Changes

When the max capacity is set to 0, cache will do the followings:

• Create the internal concurrent hash map with initial capacity of 0.
• Do not enable the frequency sketch.
• Make the following methods to do nothing and return immediately:
  • internal insert_with_hash
  • internal do_get_with_hash
  • external sync

Also added unit test called max_capacity_zero.

Verification

• Ran all unit tests and they all passed.
  • command: RUSTFLAGS='--cfg skeptic --cfg trybuild' cargo test -F future

[tatsuya6502]

Merging.