[peerdist] Limit number of concurrent raw block downloads
Raw block downloads are expensive if the origin server uses HTTPS,
since each concurrent download will require local TLS resources
(including potentially large received encrypted data buffers).
Raw block downloads may also be prohibitively slow to initiate when
the origin server is using HTTPS and client certificates. Origin
servers for PeerDist downloads are likely to be running IIS, which has
a bug that breaks session resumption and requires each connection to
go through the full client certificate verification.
Limit the total number of concurrent raw block downloads to ameliorate
these problems.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Michael Brown