Remiplement the old `ShardReplicationPositions` vtctl command with new impl
Remiplement the old `ShardReplicationPositions` vtctl command with new impl

The new implementation contains the bugfix for getting partial results
with one (or more) dead/unreachable tablets.

Closes #4073.

Signed-off-by: Andrew Mason <8a240ff75aa1467ba5eba4db0c8200b61262547e@slack-corp.com>

mahak/vitess,tinyspeck/vitess,tinyspeck/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,tinyspeck/vitess