box: expose snapshot status in box.info.gc()

Add box.info.gc.checkpoint_is_in_progress which is true when
there is an ongoing checkpoint/snapshot and false otherwise

Closes gh-3935

@TarantoolBot document
Title: box.info.gc().checkpoint_is_in_progress

Extend box.info.gc() documentation with a new member
- checkpoint_is_in_progress, which is true if there is an ongoing
checkpoint, false otherwise.