Revert "Issue #2485385 by phenaproxima, quietone, Berdir, blazey, hussainweb,...

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

This reverts commit 7c8ebc31.