Continues 2f29338 for `ObjectService`'s
`Put` server handler. opens the stage for optimizations of storage policy
imposition. Previously, node selection was hidden behind placement traverser
interface. currently, processing a single request requires preselection of
container nodes in unsorted form and, in general w/ slicing, several sorts to
place the resulting objects. In the previous code structure, cache optimization
would be less seamless. Also, this refactor reveals the entire placement logic
by removing unnecessary abstraction.

Implementation is pretty massive, so unit tests for various scenarios are
added.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

Continues 5389a1e for `ObjectService`'s
`Put` server handler. It shares the cache with other object processors.

Refs #1803.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>