Allow Collectors To "Publish" If They Can Be Used In Concurrent Search [LUCENE-8963]

There is an implied assumption today that all we need to run a query concurrently is a CollectorManager implementation. While that is true, there might be some corner cases where a Collector's semantics do not allow it to be concurrently executed (think of ES's aggregates). If a user manages to write a CollectorManager with a Collector that is not really concurrent friendly, we could end up in an undefined state.

This Jira is more of a rhetorical discussion, and to explore if we should allow Collectors to implement an API which simply returns a boolean signifying if a Collector is parallel ready or not. The default would be true, until a Collector explicitly overrides it?

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Migrated from [LUCENE-8963](https://issues.apache.org/jira/browse/LUCENE-8963) by Atri Sharma (@atris), updated Sep 05 2019

Start by reading the CollectorManager and Collector APIs and the concurrent-search behavior described in the issue. Review the discussion and related Jira context to determine whether a parallel-readiness API is wanted; the issue does not define an implementation or a clear completion condition.

Written by the indexing model from the issue text.