Add LongTermStorage class

This makes use of the new memory allocator hooks to allocate certain
objects which are expected to be long-lived in their own region of
memory. Initally this is just used for Blob objects - i.e. document
bodies held in RAM.

Change-Id: Ia8950d153a296feb29e7d66dc8c3b3d089d089be
TEMP: Disable automatic starting of defragmenterTask

Change-Id: Ic55d2b249995cd4007d91e77bcf92a590194ed6a