In current mutate_MV() code it's possible for a local endpoint
to become a target for a network operation. That's the source
of occasional `broken promise` benign error messages appearing,
since the mutation is actually applied locally, so there's no point
in creating a write response handler - the node will not send a response
to itself via network.
While at it, the code is deduplicated a little bit - with the paths
simplified, it's easier to ensure that a local endpoint is never
listed as a target for remote network operations.
Fixes#5459
Tests: unit(dev),
dtest(materialized_views_test.TestMaterializedViews.add_dc_during_mv_insert_test)
b3a6a33487