Using the pool prevents exceptions like:
```
java.nio.channels.ClosedByInterruptException: null
at java.nio.channels.spi.AbstractInterruptibleChannel.end(AbstractInterruptibleChannel.java:199) ~[?:?]
at sun.nio.ch.FileChannelImpl.endBlocking(FileChannelImpl.java:162) ~[?:?]
at sun.nio.ch.FileChannelImpl.size(FileChannelImpl.java:388) ~[?:?]
at org.rrd4j.core.RrdNioBackend.<init>(RrdNioBackend.java:94) ~[?:?]
at org.rrd4j.core.RrdNioBackendFactory.open(RrdNioBackendFactory.java:163) ~[?:?]
at org.rrd4j.core.RrdBackendFactory.getBackend(RrdBackendFactory.java:521) ~[?:?]
at org.rrd4j.core.RrdDb.<init>(RrdDb.java:627) ~[?:?]
at org.rrd4j.core.RrdDb.of(RrdDb.java:500) ~[?:?]
at org.openhab.persistence.rrd4j.internal.RRD4jPersistenceService.getDB(RRD4jPersistenceService.java:323) ~[?:?]
at org.openhab.persistence.rrd4j.internal.RRD4jPersistenceService.store(RRD4jPersistenceService.java:141) ~[?:?]
at org.openhab.persistence.rrd4j.internal.RRD4jPersistenceService.lambda$0(RRD4jPersistenceService.java:211) ~[?:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) [?:?]
at java.util.concurrent.FutureTask.run(FutureTask.java:264) [?:?]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304) [?:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
at java.lang.Thread.run(Thread.java:829) [?:?]
```
Fixes#13297
Also includes a few code improvements.
Signed-off-by: Wouter Born <github@maindrain.net>
8eb0d2c16e