We now loop over all IndexDefinitions and support online reindexing
for each of them independently. They share the single configured
batch indexing threadpool, so running an arbitrary (small) number of
them shouldn't affect performance more than running one of them.
The markReady implementation still does not correctly respect multiple
indexes; this obviously needs to be fixed before adding any new index
types.
Change-Id: Id6c58a595086e7dc22cbc7302169d96c4ccf1aa4
1762bacf0c