Each chunk of a resumable upload was buffered in a fresh RepeatableReader
which grew by appending, so bulk transfers churned up to a chunk size (10
MiB by default) of heap per chunk and GC pressure.
Buffer chunks instead with multipart.NewRW from the global page pool
instead so memory is reused across uploads and bounded by rclone's
central memory management.
A source which delivers fewer bytes than its declared size now fails
before the chunk is sent with an unexpected EOF error rather than being
rejected by the transport.
Note that accounting now happens as the chunk is read into the buffer
rather than as it is sent, as in the drive backend.
Nick Craig-Wood
689081b410