Logging brooklyn-persister thread Jclouds messages in a separate file

When a server with persistence to an object store enabled manages lots of entities the Jclouds log is overwhelmed by messages logged from the `brooklyn-persister` thread (in one case logging at a rate of ~ half a million lines every four minutes).

The appender added in this commit alleviates - a little! - this logging overload by appending messages logged by Jclouds from any thread named "brooklyn-persister" to a separate file.

Anecdotal measurements of the ratio between the persister log and jclouds log sizes, with `EmptySoftwareProcess` entities:
```
0 entities: persister log 0.01 times size of jclouds log
1 entity: 0.36
5 entities: 1.13
10 entities: 1.75
50 entities: 8.6
```

Do we want `startsWith` rather than `equals`? If there is already a thread with that name, then `return new Thread(r, "brooklyn-persister")` will use the thread name with a number suffix. That would be the case for running tests with multiple management contexts.