ZarafaBridge: logging: use proper facility

Trace is lowest, then debug, info, warn, error and fatal.

Entering a function warrants a trace statement. Performing an action is a debug
or an info statement. Errors in the "not found" category are warnings. Errors
in the "unexpected thing happened" category are errors. Errors in the "I can't
continue" category are fatal.
ZarafaCardDavBackend: log function entry at 'trace' level

Use the proper log facility for messages printed on function entry. Also avoid
hardcoding the function name; get it dynamically from __FUNCTION__.
ZarafaPrincipalsBackend: log function entry at 'trace' level
ZarafaAuthBackend: log function entry at 'trace' level