Fix prefetcher concurrent map read and write issue

[andyzhang2023]

Description

It's a bug. The triePrefetcher structure should be thread safe, and it occasionally crashes the service with fatal error of "concurrent map read and write" on the map fetchers. We just add a lock to protected the map here.

Changes

the map fetchers will be protected by an RWMutex, and there are five function calls in which the map fetchers will be accessed:

• prefetch(): read and write, need to be Lock
• trie(): read only, need to be RLock
• copy(): read only, need to be RLock
• close(): read and wirte, need to be Lock
• used(): read only, need to be RLock

[welkin22]

LGTM

[owen-reorg]

LGTM

[owen-reorg]

close this issue for now since we removed the feature in the v0.3.0 release