In Go, `http.Request` requires an `io.ReadCloser` for an upload body. To
facilitate compression of data being uploaded we want to wrap a
source reader (such an `io.File`) with a proposed
`lz4.CompressingReader`, whereupon the said reader can be set directly
into `http.Request` Body field.

This benefits not only the plain `http.Request`, but also the various
network file storage APIs which build on top of Go's http client.

Signed-off-by: Alex Dubov <[email protected]>

Out buffer may have some stale overflow data on Reset.

Signed-off-by: Alex Dubov <[email protected]>

It is possible for out buffer to have no overflow data and yet
the read offset will be not zero (if byte slices passed by the reader
just happened to align perfectly).

Signed-off-by: Alex Dubov <[email protected]>

Signed-off-by: Alex Dubov <[email protected]>