LUCENE-5145: AppendingPackedLongBuffer and added suport for bulk get operations to the Appending*Buffers.

Introduced bulk retrieval to AbstractAppendingLongBuffer
classes, for faster retrieval. Introduced a new variant,
AppendingPackedLongBuffer which solely relies on PackedInts as a backend.
This new class is useful where people have non-negative numbers with a
uniform distribution over a fixed (limited) range. Ex. facets ordinals. To
distinguish it from AppendingPackedLongBuffer, delta based
AppendingLongBuffer was renamed to AppendingDeltaPackedLongBuffer Fixed an
Issue with NullReader where it didn't respect it's valueCount in bulk gets.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1508423 13f79535-47bb-0310-9956-ffa450edef68