Optimise multi-job submit

Currently, on multi-job submits we simply iterate over the
single-job-submit function. This means we grab a new serial, write and
replicate (and wait for the remote nodes to ack) the serial file, and
only then create the job file; this is repeated N times, once for each
job.

Since job identifiers are ‘cheap’, it's simpler to simply grab at the
start a block of new IDs, write and replicate the serial count file a
single time, and then proceed with the jobs as before. This is a cheap
change that reduces I/O and reduces slightly the CPU consumption of the
master daemon: submit time seems to be cut in half for big batches of
jobs and the masterd cpu time by (I can't get consistent numbers)
between 15%-50%.

Note that this doesn't change anything for single-job submits and most
probably for < 5 job submits either.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>