During startup ActiveMq load all the scheduleDB.data on memory causing OOM

I had a broker with lots of scheduled messages and I noticed that during startup (clean or unclean) the broker was reading the whole index file and storing it os memory:

https://github.com/apache/activemq/blob/master/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/scheduler/JobSchedulerImpl.java#L665

In order to prevent the OOM, I changed this method to return a Iterator<JobLocation> instead of a List<JobLocation> avoiding load all this data into the heap.

I also noticed that during the startup we read the index at least 3 times:

https://github.com/apache/activemq/blob/master/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/scheduler/JobSchedulerStoreImpl.java#L829

https://github.com/apache/activemq/blob/master/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/scheduler/JobSchedulerStoreImpl.java#L857

and 

https://github.com/apache/activemq/blob/master/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/scheduler/JobSchedulerStoreImpl.java#L940

Probably we can optimize this but this should be another Jira (also maybe we dont need to call recover(tx) when its a clean shutdown)

https://github.com/apache/activemq/blob/master/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/scheduler/JobSchedulerStoreImpl.java#L787