Jolokia/JMX queries fail with IllegalStateException on slave broker when startAsync=true

## Problem

When a broker is configured with `startAsync=true` and is running as a slave (blocked waiting on the shared/exclusive lock), Jolokia queries against the Broker MBean fail with:

```
java.lang.IllegalStateException: Broker is not yet started.
```

Browsing the same MBean directly in JConsole shows all attributes correctly, including `Slave=true`.

## Root cause

In `BrokerView.java`, most attribute getters are safe to call before the `ManagedRegionBroker` object exists — for example `isSlave()` reads directly from `BrokerService` and never touches it. However, a few getters — `getBrokerId()`, `getBrokerName()`, plus several destination/statistics getters — call `safeGetBroker()`, which throws `IllegalStateException("Broker is not yet started.")` while the slave is still waiting on the lock.

Since Jolokia reads all attributes on an MBean in a single batch request, that one exception aborts the entire response, so attributes that would have succeeded independently (such as `Slave`) never come back either.

## Environment

- `startAsync=true`
- Broker running as slave, blocked on lock acquisition

## Suggested fix

Have the read-only getters in `BrokerView` return a safe fallback instead of throwing when the `ManagedRegionBroker` isn't set yet:
- broker name falls back to the configured name on `BrokerService`
- broker ID returns `null`
- counts return `0`
- lists return empty

Getters that actually mutate broker state (add/remove destinations, durable subscriptions, stats reset) should keep throwing, since those genuinely can't run yet.