[STORE] Simplify reading / writing from and to BlobContainer

BlobContainer used to provide async APIs which are not used
internally. The implementation of these APIs are also not async
by nature and neither is any of the pluggable BlobContainers. This
commit simplifies the API to a simple input / output stream and
reduces the hierarchy of BlobContainer dramatically.
NOTE: This is a breaking change!
Why not simply this even more by making readSnapshot to accept InputStream instead of array of bytes?