junction: rewrite BlockingCalculator to be more efficient

[pierre]

Stacktraces from the load test:

catalina-exec-3" #107 daemon prio=5 os_prio=0 tid=0x00007fc6686b5000 nid=0x401 runnable [0x00007fc5192e8000]
   java.lang.Thread.State: RUNNABLE
	at com.google.common.collect.Iterators$5.computeNext(Iterators.java:637)
	at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:141)
	at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:136)
	at com.google.common.collect.Iterators$ConcatenatedIterator.hasNext(Iterators.java:1324)
	at com.google.common.collect.Iterators$5.computeNext(Iterators.java:635)
	at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:141)
	at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:136)
	at com.google.common.collect.Iterators.addAll(Iterators.java:355)
	at com.google.common.collect.Lists.newArrayList(Lists.java:143)
	at com.google.common.collect.Lists.newArrayList(Lists.java:130)
	at org.killbill.billing.junction.plumbing.billing.BlockingCalculator.getAggregateBlockingEventsPerSubscription(BlockingCalculator.java:148)
	at org.killbill.billing.junction.plumbing.billing.BlockingCalculator.insertBlockingEvents(BlockingCalculator.java:115)
	at org.killbill.billing.junction.plumbing.billing.DefaultInternalBillingApi.getBillingEventsForAccountAndUpdateAccountBCD(DefaultInternalBillingApi.java:129)
	at org.killbill.billing.invoice.InvoiceDispatcher.processAccountWithLock(InvoiceDispatcher.java:350)
	at org.killbill.billing.invoice.InvoiceDispatcher.processAccount(InvoiceDispatcher.java:316)
	at org.killbill.billing.invoice.api.user.DefaultInvoiceUserApi.triggerDryRunInvoiceGeneration(DefaultInvoiceUserApi.java:272)

catalina-exec-3" #107 daemon prio=5 os_prio=0 tid=0x00007fc6686b5000 nid=0x401 runnable [0x00007fc5192e8000]
   java.lang.Thread.State: RUNNABLE
	at java.util.TreeMap.keyIterator(TreeMap.java:1110)
	at java.util.TreeMap$KeySet.iterator(TreeMap.java:1123)
	at java.util.TreeSet.iterator(TreeSet.java:181)
	at org.killbill.billing.junction.plumbing.billing.BlockingCalculator.filter(BlockingCalculator.java:233)
	at org.killbill.billing.junction.plumbing.billing.BlockingCalculator.insertBlockingEvents(BlockingCalculator.java:118)
	at org.killbill.billing.junction.plumbing.billing.DefaultInternalBillingApi.getBillingEventsForAccountAndUpdateAccountBCD(DefaultInternalBillingApi.java:129)
	at org.killbill.billing.invoice.InvoiceDispatcher.processAccountWithLock(InvoiceDispatcher.java:350)
	at org.killbill.billing.invoice.InvoiceDispatcher.processAccount(InvoiceDispatcher.java:316)
	at org.killbill.billing.invoice.api.user.DefaultInvoiceUserApi.triggerDryRunInvoiceGeneration(DefaultInvoiceUserApi.java:272)

catalina-exec-3" #107 daemon prio=5 os_prio=0 tid=0x00007fc6686b5000 nid=0x401 runnable [0x00007fc5192e8000]
   java.lang.Thread.State: RUNNABLE
	at java.util.HashMap.resize(HashMap.java:704)
	at java.util.HashMap.putVal(HashMap.java:629)
	at java.util.HashMap.put(HashMap.java:612)
	at org.killbill.billing.junction.plumbing.billing.BlockingCalculator.createBlockingDurations(BlockingCalculator.java:324)
	at org.killbill.billing.junction.plumbing.billing.BlockingCalculator.insertBlockingEvents(BlockingCalculator.java:116)
	at org.killbill.billing.junction.plumbing.billing.DefaultInternalBillingApi.getBillingEventsForAccountAndUpdateAccountBCD(DefaultInternalBillingApi.java:129)
	at org.killbill.billing.invoice.InvoiceDispatcher.processAccountWithLock(InvoiceDispatcher.java:350)
	at org.killbill.billing.invoice.InvoiceDispatcher.processAccount(InvoiceDispatcher.java:316)
	at org.killbill.billing.invoice.api.user.DefaultInvoiceUserApi.triggerDryRunInvoiceGeneration(DefaultInvoiceUserApi.java:272)