…astic#41418)

* Fix Watcher deadlock that can cause in-abilty to index documents.

This commit removes the usage of the `BulkProcessor` to write history documents
and delete triggered watches on a `EsRejectedExecutionException`. Since the
exception could be handled on the write thread, the write thread can be blocked
waiting on watcher threads (due to a synchronous method). This is problematic
since those watcher threads can be blocked waiting on write threads.

This commit also moves the handling of the exception to the generic threadpool
to avoid submitting write requests from the write thread pool.

fixes elastic#41390