vardbapi: disable subprocess vdb cache updates

vardbapi: disable subprocess vdb cache updates

Since commit 7535cabdf2fab76fc55df83643157613dfd66be9,
vardbapi.flush_cache() is often called within subprocesses spawned
from MergeProcess. The _aux_cache_threshold doesn't work as designed
if the cache is flushed from a subprocess like this, can lead to the
vdb cache being flushed for every single merge. This is a waste of
disk IO, so disable vdb cache updates in subprocesses.