vault: log once per interval if batching revocation

This log line should be rare since:

1. Most tokens should be logged synchronously, not via this async
   batched method. Async revocation only takes place when Vault
   connectivity is lost and after leader election so no revocations are
   missed.
2. There should rarely be >1 batch (1,000) tokens to revoke since the
   above conditions should be brief and infrequent.
3. Interval is 5 minutes, so this log line will be emitted at *most*
   once every 5 minutes.

What makes this log line rare is also what makes it interesting: due to
a bug prior to Nomad 0.11.2 some tokens may never get revoked. Therefore
Nomad tries to re-revoke them on every leader election. This caused a
massive buildup of old tokens that would never be properly revoked and
purged. Nomad 0.11.3 mostly fixed this but still had a bug in purging
revoked tokens via Raft (fixed in #8553).

The nomad.vault.distributed_tokens_revoked metric is only ticked upon
successful revocation and purging, making any bugs or slowness in the
process difficult to detect.

Logging before a potentially slow revocation+purge operation is
performed will give users much better indications of what activity is
going on should the process fail to make it to the metric.

nomad/vault.go

@@ -1271,6 +1271,9 @@ func (v *vaultClient) revokeDaemon() {
 			// Build the list of accessors that need to be revoked while pruning any TTL'd checks
 			toRevoke := len(v.revoking)
 			if toRevoke > v.maxRevokeBatchSize {
+				v.logger.Info("batching tokens to be revoked",
+					"to_revoke", toRevoke, "batch_size", v.maxRevokeBatchSize,
+					"batch_interval", v.revocationIntv)
 				toRevoke = v.maxRevokeBatchSize
 			}
 			revoking := make([]*structs.VaultAccessor, 0, toRevoke)