Replace getOperator() with getField().getName()

The index fields may be named differently then the operators supplied
by a user.  This offers some flexibility in aliasing operators like
"change" to have different backend fields based on the input string
format, or "age" being aliased to a range query on "updated".

Change-Id: Icb16b91cf7687d23694835ebaa4faf318b6af30c
1 file changed