This type alias provide cache-wrapping for Ledger objects.
Through the CachedLedger interface, access to the underlying
Ledger is permitted to allow for cases where the implementation
must perform Ledger specific activities. For example, building
a fetch pack from the contained SHAMap objects.
The CachingReadView is refactored:
* Renamed to CachedView
* Templated on Base, the base type
* base() returns a shared_ptr to the wrapped object
* Constructor requires a shared_ptr<Base>