Ensure partition drops and migrations can eliminate all competitors

The coordination of report-related garbage collection has been improved. Previously, if a separate `read-database` user were configured, incoming queries could block the garbage collection. Whenever a separate `read-database` user *is* specified, the normal (write) database user must have the right to terminate the read user's queries. The [recommended configuration]({{puppetdb}}/configure_postgres.html) has been updated to include a suitable `grant puppetdb_read to puppetdb`. The coordination of migrations has also been improved. Previously if a separate `read-database` user were configured, it wouldn't actually be evicted during migrations. Note that the migrator user (role) must have the ability to terminate the read user's connections. The [recommended configuration]({{puppetdb}}/configure_postgres.html) accomplishes this by granting the write user's role to the migrator via the `grant puppetdb to puppetdb_migrator`, which allows the migrator to terminate the read user's connections indirectly via the write user's membership in the read user's role, as described above.