Rename filter and filter_ to qfilter

We currently use 'filter' as the OpCode, QueryRequest and RAPI field
name for representing a query filter. However, since 'filter' is a
built-in function, we actually have to use filter_ throughout the code
in order to not override the built-in function.

This patch simply goes and does a global sed over the code. Due to the
fact that the RAPI interface already exposed this field, we add
compatibility code for now which handles both forms.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>