Cache client details per request

[dacrome]

Moved from https://github.com/osiam/auth-server/issues/67:

@tkrille: I've noticed that 1 request to an OAuth-protected resource on the resource server leads to several roundtrips to the client database. Seems that Spring Security OAuth calls clientDetailsService.loadClientByClientId(clientId) for almost every single attribute it has to know about. We should somehow cache the result of clientDetailsService.loadClientByClientId(clientId) on a per request basis.