If the user has wants to postpone clean-up we shouldn't lock the
indexer waiting for something that will never happen. Clear the flag
event though we are actually skipping cleanup.
Implement a new message indexer consisting of a single-threaded scanner
and a multi-threaded indexer.
This allows for a number of optimizations as well as background
indexing, though this initial version should be behave similar to the
old indexer.
