add circuit breaker for parent/child id cache

Changes include:
- Bumping the flush size for RamAccountingTermsEnum to reduce log messages when TRACE logging is enabled
- A breaker for parent/child id cache that I've tested against 20 million docs (10m parents, 10m children, 1 parent per child, parents having incremental ids & children having randomly generated ids) as well as the stackoverflow data (see graph, x axis is the segment #, y axis is kb adjustment after segment is loaded)
- Fix for a bug I discovered that made the circuit breaker revert to 80% of the heap size even if it was configured in `elasticsearch.yml`

![adj-double-term-length](https://f.cloud.github.com/assets/19060/2315476/168db0e0-a324-11e3-8400-526ee6a816e6.png)