Realtime terms dictionary [LUCENE-3245]

For #3388 we need a realtime terms dictionary. While ConcurrentSkipListMap may be used, it has drawbacks in terms of high object overhead which can impact GC collection times and heap memory usage.

If we implement a skip list that uses primitive backing arrays, we can hopefully have a data structure that is [as] fast and memory efficient.

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Migrated from [LUCENE-3245](https://issues.apache.org/jira/browse/LUCENE-3245) by Jason Rutherglen, updated Aug 29 2011
Attachments: [LUCENE-3245.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3245/LUCENE-3245.patch) (versions: 3)
Linked issues:
- [HBASE-4002](https://issues.apache.org/jira/browse/HBASE-4002)

Start by reading the migrated LUCENE-3245 description and its attached LUCENE-3245.patch, then review the context from #3388 and linked HBASE-4002. The payload names no source files or tests; done would require a primitive-array skip-list terms dictionary that addresses the stated object-overhead, GC, heap, speed, and memory-efficiency concerns.

Written by the indexing model from the issue text.