Remove caching storage in memory from `irmaclient.Client`

[w-ensink]

The following variables keep some stuff that's also on disk in memory, probably for performance reasons:

irmago/irmaclient/client.go

Lines 47 to 54 in dec8226

	// Stuff we manage on disk
	secretkey *secretKey
	attributes map[irma.CredentialTypeIdentifier][]*irma.AttributeList
	credentialsCache concmap.ConcMap[credLookup, *credential]
	keyshareServers map[irma.SchemeManagerIdentifier]*keyshareServer
	updates []update
	lookup map[string]*credLookup

Removing them and just loading them from storage when needed makes the code much simpler and less prone to bugs.
If caching turns out to be required we can always put it inside of the storage layer later. Putting it there keeps the caching logic out of the way of the general irmaclient logic.