Provider identifiers should not be required to be unique

With the introduction of Providers in 1.9, we ensured that Provider.identifier is unique. However this constraint is a mistake, especially since we have recently clarified that standard practice will be for a single human person to potentially have >1 Provider record if they work in multiple contexts, or with multiple roles, etc.

Burke says:
"At AMPATH, we would want the provider identifier to match the user system ID when creating a provider for someone who has or is getting a user account.
...
Provider identifiers should not be required to be unique. I suppose we could create some mechanism to allow them to be optionally unique (i.e., via configuration), but doing so could easily cause problems using provider data from external sources."

Mark says:
"we need to relax the restriction on provider identifiers being unique"