accounts: rename store file

We rename the `store.go` file to `store_kvdb.go` to indicate that this
file contains the kvdb implementation of the accounts DB. This is in
preparation for adding a sql-backed implementation later on.

We do this early on in the PR so that any changes that need to be made
during the review process can be easily addressed with fix-up commits
that edit the newly named file.

Author: ellemouton