Each 32 MiB chunk is its own request and opened a new SSH connection and
SFTP session (~200 ms). A worker now keeps its connection to the host for
the next chunk; each request still opens its own SFTP session (~50 ms),
so an SFTP error never reaches the next one. A connection is not reused
after an error, when the host dropped it or once it is 10 minutes old; an
idle one is closed after a minute, and a forked worker leaves the parent's
to the parent. The SFTP start, which paramiko waits for without a deadline,
is ended after 10 s. Live: 256 MiB in 8 chunks 4.5-4.9 s -> 3.7 s.
