Delete never release memory

**What is the issue you are having?**
We add data into bigcache, and using delete, but seems like it only set byte array with 0.

For example, in one big cache there are 111111111111 of a byte queue, and we use delete, it only make it to 111000111111
so next time we insert new data, it still add to the back of byte queue, rather than use those three bytes with 0.

Is there anyway to delete byte correctly so that the queue size would not grow or reuse 0 byte correctly?

For more information on how to provide an MCVE, please see the [Stack Overflow documentation](https://stackoverflow.com/help/mcve).

**Environment:**
- OS (e.g. from `/etc/os-release` or winver.exe): macosx
- go version: go1.16.8