Fixes, adjustments to numeric null handling and string first/last aggregators.

[gianm]

There is a class of bugs due to the fact that BaseObjectColumnValueSelector
has both "getObject" and "isNull" methods, but in most selector implementations
and most call sites, it is clear that the intent of "isNull" is only to apply
to the primitive getters, not the object getter. This makes sense, because the
purpose of isNull is to enable detection of nulls in otherwise-primitive columns.
Imagine a string column with a numeric selector built on top of it. You would
want it to return isNull = true, so numeric aggregators don't treat it as
all zeroes.

Sometimes this design leads people to accidentally guard non-primitive get
methods with "selector.isNull" checks, which is improper.

This patch has three goals:

1. Fix null-handling bugs that already exist in this class.
2. Make interface and doc changes that reduce the probability of future bugs.
3. Fix other, unrelated bugs I noticed in the stringFirst and stringLast
   aggregators while fixing null-handling bugs. I thought about splitting this
   into its own patch, but it ended up being tough to split from the
   null-handling fixes.

For (1) the fixes are,

• Fix StringFirst and StringLastAggregatorFactory to stop guarding getObject
  calls on isNull, by no longer extending NullableAggregatorFactory. Now uses
  -1 as a sigil value for null, to differentiate nulls and empty strings.
• Fix ExpressionFilter to stop guarding getObject calls on isNull. Also, use
  eval.asBoolean() to avoid calling getLong on the selector after already
  calling getObject.
• Fix ObjectBloomFilterAggregator to stop guarding DimensionSelector calls
  on isNull. Also, refactored slightly to avoid the overhead of calling
  getObject followed by another getter (see BloomFilterAggregatorFactory for
  part of this).

For (2) the main changes are,

• Remove the "isNull" method from BaseObjectColumnValueSelector.
• Clarify "isNull" doc on BaseNullableColumnValueSelector.
• Rename NullableAggregatorFactory -> NullbleNumericAggregatorFactory to emphasize
  that it only works on aggregators that take numbers as input.
• Similar naming changes to the Aggregator, BufferAggregator, and AggregateCombiner.
• Similar naming changes to helper methods for groupBy, ValueMatchers, etc.

For (3) the other fixes for StringFirst and StringLastAggregatorFactory are,

• Fixed buffer overrun in the buffer aggregators when some characters in the string
  code into more than one byte (the old code used "substring" to apply a byte limit,
  which is bad). I did this by introducing a new StringUtils.toUtf8WithLimit method.
• Fixed weird IncrementalIndex logic that led to reading nulls for the timestamp.
• Adjusted weird StringFirst/Last logic that worked around the weird IncrementalIndex
  behavior.
• Refactored to share code between the four aggregators.
• Improved test coverage.
• Made the base stringFirst, stringLast aggregators adaptive, and streamlined the
  xFold versions into aliases. The adaptiveness is similar to how other aggregators
  like hyperUnique work.

[vogievetsky]

Wow, that is a lot of changes. You basically rewrote them. 🚀

[fjy]

@gianm I think this is failing UT

[gianm]

I had to adjust the design somewhat to keep things working at ingest time. I added a new test for this too, to ensure it works. The branch and original description is now updated.

[lgtm-com]

This pull request introduces 1 alert when merging fe34b51 into b03aa06 - view on LGTM.com

new alerts:

• 1 for Useless null check

[clintropolis]

nice 👍

[clintropolis]

👍 this is a good change I think, moving the DimensionSelector out of here. This doesn't need to handle List since it doesn't work at ingestion time i think, and i can't think of anywhere else they would come from without a column capabilities.

[clintropolis]

👍

[clintropolis]

nit: ... limit can be changed ... or ... may be ...?