Utility method to allow empty bitmap indices to be purged after the associated
data has been purged. pChunkLimit controls the maximum number of bitmap chunks
that will be removed from the start of the extent before any populated nodes exist.
Specifying -1 will clean up ALL such empty bitmaps, and specifying 0 will not clean up any
such bitmaps. Note that bitmap chunks contain 64,000 bits, so the default value of 500 will
remove up to 500 x 64,000 bits = 32 million stale bitmap entries.
Also note that this limit ONLY applies to the main extent. Many more nodes may need to be removed
from other indices in the class, as these other indices maintain separate buckets for each possible
value. For example, a production with 100 business services would likely result in the SourceConfigName
index of the Ens.MessageHeader class having 100 distinct buckets. The bitmap purge process could then need
to remove as many as 5,000 bitmap nodes for this index alone.