Improve nats-js store

[kobergj]

We found some small issues in the nats-js store implementation:

• We ran into concurrent map writes when using natsjs store. The problem is occurring on startup when multiple go-routines try to initialize the store at the same time. We reused the existing sync.RWMutex of the store to initialize the store only once.
• Uses a sync.Map now to store the buckets in a thread safe way
• Fixes a panic when Limit is larger then the actual slice length on List
• Trims the prefix from the key when using List. Reason is that other calls (delete, read, write) add the prefix again and modifying records after listing should be possible without further modifying of the keys

[kobergj]

@jochumdev can I get a review on this?

[jochumdev]

Regarding sync.Map you might wanna have a look at cornelk/hashmap if speed matters.

I'm using hashmap in go-orb (local fork) and another small project, works fine.

[kobergj]

Thanks for the pointer 👍 Looks much better with hashmap pkg

[jochumdev]

Nice work!