storaged: remove protos from storaged class

protobuf is only needed when serializing/deserializing data. Instead of
maintaining a permanent buffer in storaged object, move the container to
stack so that the buffer is released when we don't need it. In addition,
we don't need to clear the buffer before updating it.

Also added a function to clear user io history when the user is removed.

Bug: 63740245
Change-Id: Ia5d19b9a0c3f92a93b061a56be89bb0b958a2a29