HTTP 5XX errors were being returned incorrectly from
BoltDB errors that were actually bad requests, e.g.,
names that were too long for buckets, users, and
organizations. Map BoltDB errors to correct Influx
errors and return 4XX errors where appropriate. Also
add op codes to more errors
* Move tenant.Service unit tests into its package
* Delete the top-level TenantService interface now that it's not used.
* Move helper funcs for setting up test stores into testing pkg
* Delete duplicate implementations scattered through the codebase
* Move error assertions into store-creation helpers
* refactor: migrator and introduce Store.(Create|Delete)Bucket
feat: kvmigration internal utility to create / managing kv store migrations
fix: ensure migrations applied in all test cases
* chore: update kv and migration documentation