make all accesses to the account manager kick off LoadPrefs so they can be loaded on demand
don't forget to clean up the accounts in the hash table during destruction
this change allows servers to be unique by {userid, hostname, type} combination and allows nsIMsgAccountManager::FindServer() to search for servers this way.
- remove LoadPreferences() function, make use of SetKey instead
- lazily create accounts/identities/servers are they are requested
- request pref service inside each object rather than passing the same service around
- start to support upgrading of preferences from 4.x world
- preference accessors ask prefs store directly instead of trying to maintain their own copy of each preference
- start to add support for shadow/default "fallback" account/server prefs