optimize global pinning recovery request frequency

[mortelli]

If a chunk in a large file is garbage collected, it is likely most of its chunks are garbage collected as well.

This means that we should limit the number of sent recovery requests in the case of having too many file chunks missing and in need of repair.

Conversely, the pinner should reupload the whole file if receives too many repair requests for chunks that are part of a file within a short period.