Rollup node: Admin RPC to cancel ongoing GC
What
This MR adds a new RPC GET /admin/cancel_gc which cancel any ongoing GC. It returns true if a GC was canceled.
This RPC is not exposed with the default secure ACL.
Why
It might be useful to cancel a GC if it is taking too much time. For instance for snapshot exports this allows to export the snapshot without waiting for the GC to terminate.