## Description 

Payload writes and reads can take significant time transferring
non-trivial amount of bytes. It does not make sense to do this in a
single threaded loop in `typed_store::Store`. Use `DBMap` interface for
multithreaded processing instead.

## Test Plan 

Existing unit tests.

---
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### Type of Change (Check all that apply)

- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes