17465: Synchronizes storage classes data between Collection & BlockManager.

17465: Synchronizes storage classes data between Collection & BlockManager.

Storage classes can be set at Collection instantiation time, and BlockManager
used to get that data when being instantiated by Collection, but desired
storage classes can change in the middle of a Collection instance lifetime,
and new blocks should get written on the correct classes, so the solution is
to pass a function for BlockManager to be able to query its parent Collection
instance settings at any time.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas.dipentima@curii.com>