Aggregations: Make size parsing consistent for strings.

Close #6061
Core: let Lucene kick off merges

Today, Elasticsearch has a separate merge thread pool checking once
per second (by default) if any merges are necessary, but this is no
longer necessary since we can and do now tell Lucene's
ConcurrentMergeScheduler never to "hard pause" threads when merges
fall behind, since we do our own index throttling.

This change goes back to letting Lucene launch merges as needed, and
removes these two expert settings:

  index.merge.force_async_merge
  index.merge.async_interval

Now merges kick off immediately instead of waiting up to 1 second
before running.

Closes #8643
Aggregations: Added getProperty method to Aggregations

This allows arbitrary properties to be retrieved from an aggregation tree. The property is specified using the same syntax as the
order parameter in the terms aggregation. If a property path contians a multi-bucket aggregation the property values from each bucket will be returned in an array.
[TEST] Reduce stringency of breaker assertions

While in a perfect world we should only ever have 2 circuit breaker
trips, it's possible to get a race condition between the child and the
parent breaker with many threads. Since multiple breaking exceptions are
not actually a bad thing, it's okay to relax the constraints in the
test.

The race conditions are due to no locking inside the breaker logic, to
ensure that it is as low overhead as possible. Even though no locking is
used, we use atomic counters internally to ensure that the "estimated"
numbers for the breakers are never out of sync (which this test still
checks with no leeway).