- changed Logger->SetUser() to not include brackets anymore
- changed Logger to include impersonation username in log filename when
doing per-user logging
- IPC & loopdetection are done on user#impersonation level
- changed IBackend->Logon() to include $impersontedUsername
- changed all backends and Autodiscover to use new Logon()
- changed IMPERSONATE_DELIM to '#' and moved it to Request
- chamged MAPIProvider and others to use Request::GetUser() to reflect
the user transmitting data (auth or impersonated)
- changed Request::GetAuthUser() to always return the authenticated user
- added Request::GetImpersontedUser() to always return the impersonted
user (if available)
- added Request::GetUser() to return the user operating on data
(impersonted if set - else auth).

Released under the Affero GNU General Public License (AGPL) version 3.