[storage/journal] Better read caching

[roberto-bayardo]

While journal replay is now buffered & fast, adb::any startup requires non-sequential reads of the fixed journal in order to replay an operation that updates an existing key (to ensure the mapped key isn't a collision). Because non-sequential reads such as these remain unbuffered (above the system level) the performance of this is poor.

[roberto-bayardo]

If we wish to simply speed up the db startup we could add caching there, but since there will be other non-sequential readers of state (e.g. any execution engine) it feels we should bite the bullet and solve this more generally.