Replace boltdb/bolt with etcd-io/bbolt

The 1.x line hit a bug in boltdb/bolt. Since the source repo is now archived, they had to cut a new fork to apply a fix for the bug, and switch their build over to point at the fork. 2.x can switch to [etcd-io/bbolt](https://github.com/etcd-io/bbolt/) instead, to pull in all the additional fixes they've made over the original bolt repo.

Locate the 2.x dependency declaration and build configuration that reference boltdb/bolt, then compare them with the etcd-io/bbolt module path. Update the dependency to the fork and verify that the 2.x build and existing tests complete successfully.

Written by the indexing model from the issue text.