Issue #2485385 by phenaproxima, quietone, alexpott, mikeryan, Berdir, blazey,...

Issue #2485385 by phenaproxima, quietone, alexpott, mikeryan, Berdir, blazey, xjm, hussainweb, catch, heddn, benjy: Move highwater field support to the source plugin, and do not expose its internals on MigrationInterface