Today we IW can only flush a DWPT to disk if an external resource calls flush() or refreshes a NRT reader or if a DWPT is selected as flush pending. Yet, the latter has the problem that it always ties up an indexing thread and if flush / NRT refresh is called a whole bunch of indexing threads is tied up. If IW could offer a simple `flushNextBuffer()` method that synchronously flushes the next pending or biggest active buffer to disk memory could be controlled in a more fine granular fashion from outside of the IW. This is for instance useful if more than one IW (shards) must be maintained in a single JVM / system.
LUCENE-8079 TestTopFieldCollectorEarlyTermination.testEarlyTermination() failures