Support multiple targets for a definition

[datadavev]

Related to #6

Use Case: Multiple resolution services may be deployed to handle the same level of resolution. For example, there may be multiple instances of an ARK identifier resolver that handles the redirection based on matches to NAAN records. In this case, the higher level scheme resolver (currently N2T) would select between the multiple ARK identifier resolvers and redirect the client to the selected instance.

Requires:

• Configuration change to support multiple targets. This can be through either multiple PidDefinition entries or more likely by changing the target property to a structure containing multiple targets. The latter is probably preferable since that also addresses Feature: support scheme specific identifier processing #6.
• Selection algorithm. This could be random, geo-based, or perhaps based on client / request properties.
• Service availability / status. Target selection should take into consideration of availability, especially where multiple targets exist for a definition.
• Configuration management corresponding with the configuration definition changes.