MINOR: map/acl: make "add map/acl" support an optional version number

MINOR: map/acl: make "add map/acl" support an optional version number

By passing a version number to "add map/acl", it becomes possible to
atomically replace maps and ACLs. The principle is that a new version
number is first retrieved by calling"prepare map/acl", and this version
number is used with "add map" and "add acl". Newly added entries then
remain invisible to the matching mechanism but are visible in "show
map/acl" when the version number is specified, or may be cleard with
"clear map/acl". Finally when the insertion is complete, a
"commit map/acl" command must be issued, and the version is atomically
updated so that there is no intermediate state with incomplete entries.