So far online migration only used a single thread for migration from
reviewDb to noteDb. Introduce a configuration parameter
notedb.onlineMigrationThreads to allow tuning the number of threads the
migration can use.
This helps to speedup online migration at the expense of imposing a
higher load on the Gerrit server running at the same time.
Change-Id: Icac0ec4907b82d1402330d01eb05e09fcca28682