add bettermap instead of go map

[xuetusc]

add bettermap instead of go map
delete go map key will only label it, not really delete that causes memory leak.

[hashicorp-cla]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

---

renbiao seems not to be a GitHub user.
You need a GitHub account to be able to sign the CLA.
If you have already a GitHub account, please add the email address used for this commit to your account.

_{Have you signed the CLA already but the status is still pending? Recheck it.}

[jefferai]

This code isn't thread safe. What's the memory leak exactly?

Is the benefit you see merely from swapping out the map instead of removing items?