Adds optional memory semaphore to limit the mem buffer usage in sink/source.
Note that we don't bookkeep exact, to avoid deadlock issues in higher layer.
In upload, we overlease on first buffer put to ensure we can at least fill
the desired 8M of buffers. We try to adjust when going over, but if we
fail, we fail, but at least will initiate upload -> soon release memory.
On next put, we try to grab multiples of 8M again, and so forth. Thus
potentially causing waiting for resources, without ending up not uploading
at least one active sink.
For download (source), we try to get lease for as much as we want to read,
but if we fail, we adjust this down to 256k and download anyway. Since this
will typically be released immediately, we at least don't overrun for long,
and again, avoid fully stopping, throttling rate instead.
Since, given the nature of object storage API:s, it is no more complicated to
provide a reasonable implementation of a seekable, limited, interface,
give this back, which in turn means upper layers can provide easy read-only file
interfaces. Hint hint.
