Resolves the specified variable. This implementation will try to extract
a variable prefix from the given variable name (the first colon (':') is
used as prefix separator). It then passes the name of the variable with
the prefix stripped to the lookup object registered for this prefix. If
no prefix can be found or if the associated lookup object cannot resolve
this variable, the default lookup object will be used.
event - The current LogEvent or null.
var - the name of the variable whose value is to be looked up
the value of this variable or null if it cannot be