Watcher uses a connection pool for outgoing HTTP traffic, which means
that some HTTP connections may live for a long time, possibly in an idle
state. Such connections may be silently torn down by a remote device, so
that when we re-use them we encounter a `Connection reset` or similar
error.
This commit introduces a setting allowing users to set a finite expiry
time on these connections, and also enables TCP keepalives on them by
default so that a remote teardown will be actively detected sooner.
Closes#52997
3e0959f308