externalBlockStoreInitialized is never set to be true, which causes the blocks stored in ExternalBlockStore can not be removed.

Author: Mingfei <mingfei.shi@intel.com>

Closes #6702 from shimingfei/SetTrue and squashes the following commits:

add61d8 [Mingfei] Set externalBlockStoreInitialized to be true, after ExternalBlockStore is initialized