Add support for parallel checkpoint ops with -jcp/-jdp/-jrp.
When parallel checkpoints are added to p4d, the SDP will need
to adapt to expose parallel checkpoint feature (with the usual
$P4D_VERSION check to only attempt to use it if the p4d version
is new enough to support it).
To define the number of threads, supply the -N option and/or
interact with the db.checkpoint.threads configurable.