Fix ever-growing accounts map

[brennanjl]

As discussed, there is a map that was added to the account store that grows forever. We should make sure to use a proper LRU cache or clear it periodically (e.g. at the end of each block) to make sure that we don't have it grow indefinitely.

[jchappelow]

The hashicorp LRU package is a good option, as is https://pkg.go.dev/github.com/decred/dcrd/container/lru, which is written by some very good developers in the space.