MB-7685: Refactor the VKeyStatBGFetchCallback task

Change-Id: I9f1db236cd0f2423a27c8e3acaa57c6242ef2b41
MB-7685: Refactor BGFetchCallback task

Change-Id: Ide033f5469a5aaa2f9efc870bb290a63299d84ba
Add new iomanager

Change-Id: Iccaa2c027cdb9abdd6c580973f94ee6d254ef035
Move the BgFetcher to the new dispatcher

Change-Id: Ia07817bfff685134d6732afda09d47ee5c065a05