Share functionality of placement building between components

[cthulhu-rider]

Object placement is build using netmap.Source and container.Source in most scenarios. Also there are some similar cases when we use same placement approach, e.g. shard evacuation introduced here and remote placement builder.

I propose to concentrate all placement functionality in single shared code space in order to easily reuse it in new tasks.

[fyrchik]

With a proper interface, tree server can use this component too.