* migrate PersistentIdentityService to use key.toShortString()
update definition of PublicKeyToExternalId mapping to allow fast lookup by externalId/publicKey
* fix misspelled table name
* add test of migration script
* add design document for proposal to move IdentityService to using the correct PK.toStringShort() method for hashing a publickey
* add enterprise testing considerations to design
* address review comments
* fix compilation errors
* modify PublicKeyToOwningIdentityCache to use toStringShort() as it's lookup key
* address syzmon's code review comments
