The rejected execution handler API says that rejectedExecution(Runnable,
ThreadPoolExecutor) throws a RejectedExecutionException if the task must
be rejected due to capacity on the executor. We do throw something that
smells like a RejectedExecutionException (it is named
EsRejectedExecutionException) yet we violate the API because
EsRejectedExecutionException is not a RejectedExecutionException. This
has caused problems before where we try to catch RejectedExecution when
invoking rejectedExecution but this causes EsRejectedExecutionException
to go uncaught. This commit addresses this by modifying
EsRejectedExecutionException to extend
RejectedExecutionException.
* Decouple XContentBuilder from BytesReference
This commit removes all mentions of `BytesReference` from `XContentBuilder`.
This is needed so that we can completely decouple the XContent code and move it
into its own dependency.
While this change appears large, it is due to two main changes, moving
`.bytes()` and `.string()` out of XContentBuilder itself into static methods
`BytesReference.bytes` and `Strings.toString` respectively. The rest of the
change is code reacting to these changes (the majority of it in tests).
Relates to #28504
