casync-http: add support for cURL multi interface

This commit brings parallel downloads to casync-http, by using the cURL
multi interface, and more precisely the "select()" flavour. For details
see https://curl.haxx.se/libcurl/c/libcurl-multi.html.

The cURL library has two ways to achieve parallel downloads:

    for HTTP/1, it can open parallel connections. The maximum number of
    parallel connections is user-defined, through MAX_HOST_CONNECTIONS
    and MAX_TOTAL_CONNECTIONS.

    for HTTP/2, it can attempt to multiplex on a single connection. The
    maximum number of parallel downloads in this case is negociated
    between the client and the server (we talk about number of streams in
    the HTTP/2 jargon).

Note that libcurl used to do pipelining over HTTP/1.1, but this is no
longer supported since 7.62.0, and casync-http doesn't use it anyway)

Signed-off-by: Arnaud Rebillout <[email protected]>
Signed-off-by: Gaël PORTAY <[email protected]>

Author: elboulangero