Serialises Mob.DNS.resolve/1's writes to :inet_db's runtime host table.

Replacing one host's entry is a read-modify-write spanning several :inet_db calls, so two resolves sharing an address would otherwise drop each other's names. A locally registered process is the lock: :global.trans/3 keys its release on the node name captured at acquire time, and Mob.Dist renames the node when it starts distribution after boot, which leaves that lock held forever. Started on first use and unlinked, because mob has no supervision tree of its own and a resolve from a screen callback must not tie the seeder's life to that screen.