Task manager: every task type follows the node's cluster role

A 3-node rehearsal found taskQueueProcessing didn't filter anything:
roles.task_manager only decided whether the task manager started, and
report, ACME, DKIM, DNS, calendar, thread-merge and restore tasks ran on
any node with a task manager (manager.rs returned true for them). A node
whose role left taskQueueProcessing off still ran them if it indexed or
did maintenance.

Every task type now answers to one ClusterTaskType (task_enabled):

- IndexDocument, UnindexDocument, IndexTrace: searchIndexing
- AccountMaintenance, TenantMaintenance, DestroyAccount:
  accountMaintenance
- StoreMaintenance: storeMaintenance
- SpamFilterMaintenance: spamClassifierTraining
- DmarcReport, TlsReport: outboundMta. They build and send reports to
  other domains (TLS reports can go straight to an HTTPS endpoint),
  which is the outbound MTA's business.
- CalendarAlarmEmail, CalendarAlarmNotification, CalendarItipMessage,
  MergeThreads, RestoreArchivedItem, AcmeRenewal, DkimManagement,
  DnsManagement: taskQueueProcessing, the role for queue tasks with no
  role of their own.

A node that may not run a task leaves it unclaimed (no lock), so a node
that may picks it up. The task manager also starts on a node whose only
task role is outboundMta, so reports still run there.

cluster::task_roles::task_role_tests (new, two task managers over one
PostgreSQL store): node A (taskQueueProcessing only) runs a DNS task and
leaves an unindex task and a TLS report pending; node B (searchIndexing
and outboundMta) comes up and runs those two; a DNS task scheduled next
stays pending on B and runs on A. On main node A runs the TLS report.

tests/src/cluster/mod.rs

@@ -10,3 +10,4 @@ pub mod broadcast;
 #[cfg(feature = "nats")]
 pub mod coordinator; // inbuxa: coordinator reconnects
 pub mod stress;
+pub mod task_roles; // inbuxa: task types follow cluster roles