replace sync.Map

[dustinxie]

What would you like to be added:

Why is this needed:
golang/go#40999

there are 2 issues:

1. go 15.1 introduced a regression, that keys in sync.Map.dirty are not deleted
2. even in go 1.14, keys in sync.Map.read are not purged immediately, see comments at the end:

(But also bear in mind that sync.Map is intended to solve a specific pattern in the Go standard library. It is surely not optimal for all “concurrent map” use-cases, and if it's not a good fit for your use-case it is totally fine to use something else instead. sync.Map should almost never appear in exported APIs, so in most cases it is easy to swap out for something else.)

[dustinxie]

let's replace all use of sync/map with "github.com/iotexproject/go-pkgs/cache"