[migractions] Better handle removal of RQLConstraint in sync_schema In rdef synchronisation, filter unmodified constraints before processing the sets of old and new constraints. Add a new `constraint_by_eid` method on RelationDefinitionSchema, which eliminates the ambiguity of `constraint_by_type` usage when several constraints of the same type are involved in the same migration. Closes #3633644.

[test] when the config change, we should cleanup sys.path and sys.modules. Closes #3634164 Without this, modules imported within instance home will be messed up.

[wsgi] make sure request.content is available for consumption The wsgi.input stream is not seekable, so make a copy either to memory or disk so it's still available to the user even after we've parsed it (for POST form processing). Closes #3554996