Fix for zero-sized content throwing off toChannelBuffer(). Also
short-circuit writeTo(..) accordingly to avoid unnecessary work.

Fixes #5543
Capture and set start time in Delete By Query operations

This is important for queries/filters that use `now` in date based queries/filters

Closes #5540
Assorted fixes for bugs in the PagedBytesReference tests, plus a typo
where the wrong variable was used for a calculation.

Closes #5549
[DOCS] fixed typo

Closes #5272
[TEST] Moved wipe* methods, randomIndexTemplate & ensureEstimatedStats from ElasticsearchIntegrationTest to TestCluster

This is the first to make it possible to have a different impl of TestCluster (e.g. based on an external cluster) that has the same methods but a different impl for them (e.g. it might use the REST API to do the same instead of the Java API)

Closes #5542