"AsyncCacheWriter is dead!" errors cause deadlock or unbounded use of memory

Caused by a relatively small AsyncStore shutdownTimeout (default 25s) in conjunction with large queue sizes or slow back-end stores.

With assertions enabled, the coordinator thread will exit with AssertionError and the AsyncStore will deadlock, as no more changes are beeing processed.

With assertions disabled, the coordinator thread will allocate an unbounded number of AsyncStoreProcessors. If AsyncStoreProcessors were multithreaded (see ISPN-3532), this would cause loss of data (due to older changes overwriting newer changes in the back-end store).