journald: make sure we retain all stream fds across restarts (#6348)

Currently we set 4096 as maximum for number of stream connections that
we accept. However maximum number of file descriptors that systemd is
willing to accept from us is just 1024. This means we can't retain all
stream connections that we accepted. Hence bump the limit of fds in a
unit file so that systemd holds open all stream fds while we are
restarted.

New limit is set to 4224 (4096 + 128).
(cherry picked from commit 3c978aca)