Add optional in-memory cache for use with exchange records

[dlongley]

To better support OID4* flows which require additional configuration information to be sent about a particular exchange, implement a LRU cache for exchange records. Pre-warm the cache with any newly created exchanges.

This cache should be used when getting well-known oauth config metadata -- and optionally (and optimistically) in other places, but care must be taken not to waste cycles on multistep exchanges with changing state that will require fresh records anyway, etc.