mgmtd: normalize argument order to copy(dst, src)

mgmtd: normalize argument order to copy(dst, src)

Having just completed a code audit during RCA, the fact that we have 2
different argument orders for the related datastore copying functions
was unnecessary and super confusing.

Fix this code-maintenance/comprehension mistake and move the newer mgmtd
copy routines to use the same arg order as the pre-existing underlying
northbound copy functions (i.e., use `copy(dst, src)`)

Signed-off-by: Christian Hopps <chopps@labn.net>