ColumnStatistics on Previewed Column May throw IllegalArgumentException

[nbauernfeind]

Simply groovy:

dh_table = emptyTable(10000).update(
    "someDateColumn = i % 10 == 0 ? null : java.time.LocalDate.ofEpochDay(i)",
    "nullDateColumn = (java.time.LocalDate)null",
    "someTimeColumn = i % 10 == 0 ? null : java.time.LocalTime.of(i%24, i%60, (i+10)%60)",
    "nullTimeColumn = (java.time.LocalTime)null"
)

(P.S. yes, I stole this from Jianfeng's writeup of #4823)

Then, however over any of the four columns to get this wicked IllegalArgumentException:

Internal Error '5bca3466-9f64-48f5-923e-a30189d8a57d' java.lang.IllegalArgumentException: Equal objects must have equal hashcodes. During rehashing, Trove discovered that the following two objects claim to be equal (as in java.lang.Object.equals()) but their hashCodes (or those calculated by your TObjectHashingStrategy) are not equal.This violates the general contract of java.lang.Object.hashCode().  See bullet point two in that method's documentation. object #1 =class io.deephaven.engine.table.impl.preview.DisplayWrapper id= 1391026345 hashCode= 1391026345 toString= 00:24:34; object #2 =class io.deephaven.engine.table.impl.preview.DisplayWrapper id= 1968006525 hashCode= 1968006525 toString= 00:24:34

	at gnu.trove.impl.hash.TObjectHash.buildObjectContractViolation(TObjectHash.java:464)
	at gnu.trove.impl.hash.TObjectHash.throwObjectContractViolation(TObjectHash.java:426)
	at gnu.trove.map.hash.TObjectLongHashMap.rehash(TObjectLongHashMap.java:191)
	at gnu.trove.impl.hash.THash.postInsertHook(THash.java:388)
	at gnu.trove.map.hash.TObjectLongHashMap.adjustOrPutValue(TObjectLongHashMap.java:456)
	at io.deephaven.server.table.stats.ObjectChunkedStats.processChunks(ObjectChunkedStats.java:55)
	at io.deephaven.server.table.ops.ColumnStatisticsGrpcImpl.lambda$create$0(ColumnStatisticsGrpcImpl.java:106)
	at io.deephaven.engine.table.impl.remote.ConstructSnapshot.callDataSnapshotFunction(ConstructSnapshot.java:1193)
	at io.deephaven.engine.table.impl.remote.ConstructSnapshot.callDataSnapshotFunction(ConstructSnapshot.java:1169)
	at io.deephaven.server.table.ops.ColumnStatisticsGrpcImpl.create(ColumnStatisticsGrpcImpl.java:102)
	at io.deephaven.server.table.ops.ColumnStatisticsGrpcImpl.create(ColumnStatisticsGrpcImpl.java:32)
	at io.deephaven.server.table.ops.TableServiceGrpcImpl.lambda$oneShotOperationWrapper$9(TableServiceGrpcImpl.java:629)
	at io.deephaven.server.session.SessionState$ExportObject.doExport(SessionState.java:974)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
	at io.deephaven.server.runner.scheduler.SchedulerModule$ThreadFactory.lambda$newThread$0(SchedulerModule.java:78)
	at java.base/java.lang.Thread.run(Thread.java:832)

I suspect we shouldn't generate stats for a previewed column - or if we do, do it on the original column (but probably not do it at all?).

[niloc132]

Alternatively, the various PreviewType impls could delegate their hashcode/equals impls to the underlying object. Fixing #188 and its related subscription/preview cleanup will also resolve this.