do not automatically delete state-persisting instruments when they are removed from the bigband file

[imaman]

auto-deletion of lambda functions is fine because you can easily rollback by means of source-control-reverting. However, for state-persisting instruments (dynamodb, s3) this is not the case. once auto-deleted was carried out, we cannot restore the data, and it is effectively lost.

we need to protect user from accidental deletion of state-persisting instruments. Two directions that come to mind:

• a special command line flag to signal the user's explicit will to delete such instruments (probably the flag should explicitly list the names of the instruments to be deleted)
• a special limbo zombie instrument to replace the actual instrument. when such a bigband is shipped deletion will occur. Otherwise, removal of a (non-zombie) state-persisting instrument will not result in an actual deletion.